Add support for decompressing ETC2

[BlueCube3310]

Fixes #25595

TODO:

• Add editor-only build macros,
• Add boundary checks for the decompressed buffer,
• Verify the decompressed data

[fire]

I don't have a setup for etc2 verification right now. I recall these types of PRs require device access.

[BlueCube3310]

It's possible to test this by enabling 'import as etc2/astc' in the project settings and copying the *.etc2.ctex file from the project's imported directory into the main one

[BlueCube3310]

I've added boundary checks (in a similar way to bcdec), it should function correctly now

[BlueCube3310]

Here are some etc2 textures for testing purposes:
tex.zip

On hardware that supports etc2, they will be passed directly to the gpu, while on ones that don't they will be decompressed

[Calinou]

Tested locally using the textures from tex.zip, it works as expected.

Testing project: test_etc2_load.zip

What's interesting is that the texture type appears to be ETC2, with the memory usage displayed as if the texture was uncompressed RGBA8 (even if the source ETC2 texture is RGB8 without alpha). This is on a NVIDIA GPU where ETC2 textures aren't natively supported:

Does this mean the texture is uncompressed to RGBA8 instead of RGB8 even if it has no alpha channel?

Uncompressed RGB8	Uncompressed RGBA8

[BlueCube3310]

Does this mean the texture is uncompressed to RGBA8 instead of RGB8 even if it has no alpha channel?

Yes, etcpak (and many more block decompression tools) work this way to simplify the alignment to 4 bytes, though when using Vulkan most devices don't support directly sampling from RGB8 textures anyway, so they still need to be converted to RGBA8

[akien-mga]

Looks good to me, assuming the size impact is reasonable.

[akien-mga]

We should measure the size impact on a template_release build to make sure that the increase in binary size is worth the added feature.

[akien-mga]

I made some template_release builds with the official build scripts to evaluate the size impact.

-rw-r--r--. 1 akien akien 9450452 Jan 13 21:32 out-master/web/templates/godot.web.template_release.wasm32.zip
-rw-r--r--. 1 akien akien 9453583 Jan 13 21:01 out-pr-100365/web/templates/godot.web.template_release.wasm32.zip

-rwxr-xr-x. 1 akien akien 69352248 Jan 13 21:32 out-master/linux/x86_64/templates/godot.linuxbsd.template_release.x86_64
-rwxr-xr-x. 1 akien akien 69376824 Jan 13 21:01 out-pr-100365/linux/x86_64/templates/godot.linuxbsd.template_release.x86_64

-rwxr-xr-x. 1 akien akien 86819328 Jan 13 21:32 out-master/windows/x86_64/templates/godot.windows.template_release.x86_64.exe
-rwxr-xr-x. 1 akien akien 86844416 Jan 13 21:01 out-pr-100365/windows/x86_64/templates/godot.windows.template_release.x86_64.exe

The size impact seems really minimal:

• +3 kB for the Web template zip (+0.033%)
• +24.5 kB for the Linux template (+0.035%)
• +25 kB for the Windows template (+0.029%)

That seems somewhat surprising, but I suppose we only use very little code in the template so the rest gets optimized out by the linker.

[akien-mga]

Thanks!