CompressedTexture: Add more ASTC formats.

[Mugen87]

Fixed #18764.

@YorickCobb 37847 should now be recognized as SRGB8_ALPHA8_ASTC_8x8_Format. Does the PR look good to you?

[YorickCobb]

Works perfect 👍

[donmccurdy]

How do the sRGB formats work in conjunction with texture.encoding? Would you also set texture.encoding = THREE.sRGBEncoding on a texture using SRGB8_ALPHA8_ASTC_8x8_Format, or is the sRGB->linear step done by WebGL?

[YorickCobb]

You still have to set texture.encoding for it to render correctly, but I humbly suggest that this is the correct behavior. AFAIK sRGB-encoded PNGs for example still default to linear encoding, so it would be inconsistent if ASTCs/any other specific format didn't also respect the default. The advantage in sRGB-encoded textures is that the bit depth is spread out more usefully, so this doesn't make the distinction useless or anything.

[Mugen87]

or is the sRGB->linear step done by WebGL?

EXT_sRGB is for this use case. In WebGL 2 you can use the respective formats without the extension.

When defining the internal format of a texture as e.g. gl.SRGB8, the texture will be automatically converted to linear space when being fetched in the shader. This is of course more performant than doing the color space conversion manually.

[mrdoob]

Thanks!