OpenGL: Unconditionally do glDisable(GL_FRAMEBUFFER_SRGB) because we do our own sRGB conversion

[dsnopek]

Fixes #95313

I think this is the right thing to do, because:

• OpenGL will only do the linear to sRGB conversion enabled via glEnable(GL_FRAMEBUFFER_SRGB) on FBO's that have a color attachment which uses an sRGB internal format. So, it will do the conversion on GL_SRGB8_ALPHA8, but not on GL_RGBA8
• Godot's renderer doesn't normally use any sRGB internal formats for render buffers, because we do the linear to sRGB conversion ourselves in our shaders
• We're only using GL_SRGB8_ALPHA8 in OpenXR, because if we don't, the OpenXR compositor will assume the content is linear, and it'll do another linear to sRGB conversion. So, we're forced to use that as the internal format, in order to signal that we're giving it sRGB data
• It should be safe to do glDisable(GL_FRAMEBUFFER_SRGB) unconditionally, because Godot's OpenGL renderer isn't even relying on that anyway (as explained above in my second point in this list)

The main thing I'm not sure about, is if doing this in RasterizerGLES3::begin_frame() is the right place to do it? We don't actually need to do it every frame, but I couldn't find an obvious central place to do it, It works fine in GLES3::Config::Config() which only runs once, but we're only querying config there, not setting it, so it seemed inappropriate. I'd appreciate any advice about this!

UPDATE: Moved to RasterizerGLES3::RasterizerGLES3() based on advice from Clay.

This PR also partially reverts the changes from #74892, which did glDisable(GL_FRAMEBUFFER_SRGB), but only when rendering viewports with use_xr set to true (which doesn't help the composition layers). With this PR doing it unconditionally, that is no longer necessary.

[clayjohn]

Looks great to me!

[decacis]

I can confirm this fixes #95313 I tested both on Quest native and with Quest Link.

4.3-rc3	This PR

And it's supposed to look like this PR :)

[devloglogan]

Tested this out by making a few changes/comparisons in the meta composition layer sample, looks good to me!

[akien-mga]

Thanks!

[akien-mga]

Cherry-picked for 4.3.1.