WebGL2: Fix 2D array textures with RGTC compression not rendering

[BlueCube3310]

Fixes #101711

WebGL2 seemingly doesn't support 2D array RGTC-compressed textures (though it's not documented anywhere). This PR makes it so that under WebGL2 layered textures with RGTC compression are decompressed before being used.

The memory usage increase shouldn't be too significant, since the decompressed images only use either 1 or 2 color channels, so they will be twice as big (8 bytes vs 16 for RGTC_R and 16 bytes vs 32 for RGTC_RG)

[Calinou]

Makes sense to me.

[clayjohn]

WebGL uses the EXT_texture_compression_rgtc extension to support RGTC, which only supports 2D images (not 3D images which include array textures).

Compare that to the WEBGL_compressed_texture extension (for ETC) which specifically mentions array textures.

Implementations are required to support either ETC or S3TC/RGTC, but the wording of both the RGTC and S3TC extensions don't cover array textures. Therefore they are not required to be supported for RGTC or S3TC.

Have you checked whether S3TC array textures are supported on your device? I wonder if there is another optional extension that adds support that we could check for. The extension for S3TC mandates support for 3D textures EXT_texture_compression_s3tc. So there is nothing to worry about

Edit: The plot thickens. The RGTC extension says that it is basically just EXT_texture_compression_rgtc which specifically says that it errors with 1D or 3D texture types (this is true on any mobile device as well). The extensions for BPTC and ASTC have similar wording (in the sense that they only mention 2D variants, but refer to a different extension for more details). But in both cases the extension they refer to specifically includes 3D textures and/or texture arrays

[clayjohn]

On an unrelated note, it would be interesting to look into properly supporting ASTC in the web. I think all we need to do is check for WEBGL_compressed_texture_astc and then it should work just like BPTC. Also it seems very widely support so for web exports we may want to always use ASTC when available

[clayjohn]

After looking into it, this seems to be true for all GLES3 contexts since they rely on EXT_texture_compression_rgtc which explicitly does not support 3D textures.

Therefore, I think this needs to use is_gles_over_gl() instead to capture any time we use a GLES3 context (i.e. web, mobile, and ANGLE on desktop).

I am not 100% sure about ANGLE on desktop though, it might work anyway despite not being guaranteed by the spec

[BlueCube3310]

On an unrelated note, it would be interesting to look into properly supporting ASTC in the web. I think all we need to do is check for WEBGL_compressed_texture_astc and then it should work just like BPTC. Also it seems very widely support so for web exports we may want to always use ASTC when available

Added in #101932

[Repiteo]

Thanks!