WebGLRenderer: Auto-detect sRGB compressed texture formats.

[Mugen87]

Related issue: see #23116 (comment)

Description

Second attempt of integrating sRGB compressed texture formats. Compared to #23116 this PR automatically detects based on Texture.format and Texture.encoding whether sRGB compressed texture formats should be used or not. E.g. a texture with this setup...

texture.format = THREE.RGBA_S3TC_DXT5_Format;
texture.encoding = THREE.sRGBEncoding;

... will be internally translated to COMPRESSED_SRGB_ALPHA_S3TC_DXT5_EXT (instead of COMPRESSED_RGBA_S3TC_DXT5_EXT).

[Mugen87]

In order to prevent a double sRGB decode, WebGLPrograms was modified to disable the inline decode. This is something we have to do anyway so I guess this is a good opportunity.

[donmccurdy]

Thanks! I like this solution. I'm not sure I understood this part of your earlier comment:

We need an enhancement in WebGLTextures so we can translate the format for both WebGL1 and WebGL 2.

... do we still need some WebGL1-specific workaround here?

[Mugen87]

... do we still need some WebGL1-specific workaround here?

The idea is to handle the conversion in WebGLUtils. It works then for WebGL 1 and WebGL 2.

[mrdoob]

Thanks!