Additional texture pixel formats [BC3, BC7, ETC2 with sRGB, ASTC 4x4]

[allcreater]

This request is adding support for some broadly used compressed texture formats, which could be very useful with the basis_universal texture transcoding library.

Extended set of "known" formats with SRGB support:

• BC3_SRGBA
• BC7_SRGBA
• ETC2_SRGB8
• ETC2_SRGB8A8

"Brand-new" ASTC format:

• ASTC_4x4_RGBA,
• ASTC_4x4_SRGBA

4x4 subset is selected as the most broadly used and the only one of ASTC formats that is supported by the transcoding library.

[floooh]

Github Actions being wonky again :/ I just restarted the failed iOS job and it worked (previously it was failing to pull in a git submodule).

Ah, I was wondering exactly why only select the 4x4 block size for ASTC... thanks for pointing that out :)

I'll need to wrap up some work on the sokol-zig bindings (hopefully tomorrow evening) then I can look into your PR.

[floooh]

Giving it a quick test now...

[floooh]

And merged. Thanks!

Only thing I changed was moving the SRGB pixel formats closer to their non-SRGB equivalents in the sg_pixel_format enum, and a small if-else coding style change.