Fix GLES3 gaussian_blur mipmap setup.

[mooflu]

Fixes #91717

Extend max level to include i for writing and so fb is complete and avoid resulting errors like:
"Framebuffer is incomplete: Attachment level is not in the [base level, max level] range".

Removed redundant glBindTexture and moved mipmap level config outside of loop. Main bug was that base and max were set to 'i-1' but glFramebufferTexture2D used 'i' resulting in errors like: "Framebuffer is incomplete: Attachment level is not in the [base level, max level] range".

Tested with --display-driver windows --rendering-driver opengl3_angle and web build.

[manuq]

I can confirm that this fixes a bug I'm seeing in my project. I have a fragment shader that deforms UV so it looks like the player sinks the floor. In the editor on Linux it looks fine, but in web export there is a darkness that should be transparent:

And the following error appears multiple times in the browser console:

GL_INVALID_FRAMEBUFFER_OPERATION: Framebuffer is incomplete: Attachment level is not in the [base level, max level] range.

You can check for yourself in https://endlessm.github.io/threadbare/branches/endlessm/soft-floor-2/ (shader source code here).

After downloading the web export template from this PR artifacts, and exporting the project with it, I can see the bug is gone:

[clayjohn]

I need to double check the OpenGL specification. But I think it is supposed to be an error to read and write to the same texture unless the level bound by glFramebufferTexture2D is outside the range set by GL_TEXTURE_BASE_LEVEL and GL_TEXTURE_MAX_LEVEL

edit: Indeed, this PR introduces undefined behaviour from the spec:

Rendering Feedback Loops
The mechanisms for attaching textures to a framebuffer object do not prevent a
one-or two-dimensional texture level, a face of a cube map texture level, or a layer
of a two-dimensional array or three-dimensional texture from being attached to the
draw framebuffer while the same texture is bound to a texture unit. While this
conditions holds, texturing operations accessing that image will produce undefined
results, as described at the end of section 3.8.11. Conditions resulting in such
undefined behavior are defined in more detail below. Such undefined texturing
operations are likely to leave the final results of fragment processing operations
undefined, and should be avoided.

Special precautions need to be taken to avoid attaching a texture image to the
currently bound framebuffer while the texture object is currently bound and enabled for texturing. Doing so could lead to the creation of a rendering feedback
loop between the writing of pixels by GL rendering operations and the simultaneous reading of those same pixels when used as texels in the currently bound
texture. In this scenario, the framebuffer will be considered framebuffer complete
(see section 4.4.4), but the values of fragments rendered while in this state will be
undefined. The values of texture samples may be undefined as well, as described
under “Rendering Feedback Loops” in section 3.8.11

Specifically, the values of rendered fragments are undefined if all of the following conditions are true:
• an image from texture object T is attached to the currently bound draw framebuffer at attachment point A
• the texture object T is currently bound to a texture unit U, and
• the current programmable vertex and/or fragment processing state makes it possible (see below) to sample from the texture object T bound to texture unit U

while either of the following conditions are true:
• the value of TEXTURE_MIN_FILTER for texture object T is NEAREST or
LINEAR, and the value of FRAMEBUFFER_ATTACHMENT_TEXTURE_LEVEL
for attachment point A is equal to the value of TEXTURE_BASE_LEVEL for
the texture object T
• the value of TEXTURE_MIN_FILTER for texture object T is one
of NEAREST_MIPMAP_NEAREST, NEAREST_MIPMAP_LINEAR, LINEAR_-
MIPMAP_NEAREST, or LINEAR_MIPMAP_LINEAR, and the value of
FRAMEBUFFER_ATTACHMENT_TEXTURE_LEVEL for attachment point A is
within the the range specified by the current values of TEXTURE_BASE_-
LEVEL to q, inclusive, for the texture object T. (q is defined in the Mipmapping discussion of section 3.8.11).

For the purpose of this discussion, it is possible to sample from the texture
object T bound to texture unit U if the active fragment or vertex shader contains
any instructions that might sample from the texture object T bound to U, even if
those instructions might only be executed conditionally.

Note that if TEXTURE_BASE_LEVEL and TEXTURE_MAX_LEVEL exclude any
levels containing image(s) attached to the currently bound framebuffer, then the
above conditions will not be met (i.e., the above rule will not cause the values of
rendered fragments to be undefined.)

[mooflu]

@clayjohn Thanks for review.

If I read the spec right, changing the original code from

		glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_BASE_LEVEL, i - 1);
		glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAX_LEVEL, i - 1);
		glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, p_source_texture, i);

to

		glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_BASE_LEVEL, i - 1);
		glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAX_LEVEL, i);
		glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, p_source_texture, i);

should be ok.

It allows for glFramebufferTexture2D to attach level i for writing and be complete.
Since the blur shader uses lod of 0.0 in the textureLods it would only access the base level i-1 for reading.

With this neither of the conditions are met that would make behavior undefined

...while either of the following conditions are true:
• the value of TEXTURE_MIN_FILTER for texture object T is NEAREST or
LINEAR, and the value of FRAMEBUFFER_ATTACHMENT_TEXTURE_LEVEL
for attachment point A is equal to the value of TEXTURE_BASE_LEVEL for
the texture object T
• the value of TEXTURE_MIN_FILTER for texture object T is one
of NEAREST_MIPMAP_NEAREST, NEAREST_MIPMAP_LINEAR, LINEAR_-
MIPMAP_NEAREST, or LINEAR_MIPMAP_LINEAR, and the value of
FRAMEBUFFER_ATTACHMENT_TEXTURE_LEVEL for attachment point A is
within the the range specified by the current values of TEXTURE_BASE_-
LEVEL to q, inclusive, for the texture object T. (q is defined in the Mipmapping discussion of section 3.8.11).

i.e. TEXTURE_MIN_FILTER is GL_LINEAR (so first bullet), but FRAMEBUFFER_ATTACHMENT_TEXTURE_LEVEL is not set to base

[clayjohn]

@mooflu Good point! In my head we were using LINEAR_MIPMAP_LINEAR so the second condition applied. But since we are using plain GL_LINEAR we are okay.

Thanks for clearing that up.

[clayjohn]

Looks great! I didn't realize earlier, but with the last change we always read from mip level 0 which degraded the quality of the blur. This change fixes the error and maintains the same quality!

In theory, we could probably set GL_TEXTURE_MAX_LEVEL outside the loop (and avoid resetting it at the end of the function) since we only need to care about the base level, but I am happy either way

edit: note for maintainers, I confirm this is good to cherrypick to earlier versions

[Repiteo]

Thanks!

[akien-mga]

Cherry-picked for 4.4.1.