shrinking wasm for glTF extension support

[andreasplesch]

We are exploring supporting the glTF_texture_basisu extensions without requiring heavy wasm files.

The extension requires support for these internal formats:

• ETC1S with BasisLZ (used for color-perceptual content)
• UASTC with optional Zstandard compression (used for data content)

Since ASTC seems to be the most widely (chrome/firefox/safari/android/ios) GPU compressed format, we would only target ASTC, and require fallback textures if not available.

https://github.com/BinomialLLC/basis_universal/wiki/How-to-Use-and-Configure-the-Transcoder#shrinking-the-transcoders-compiled-size

provides options to select for compilation:
BASISD_SUPPORT_UASTC
BASISD_SUPPORT_ASTC or BASISD_SUPPORT_ASTC_HIGHER_OPAQUE_QUALITY
plus BASISD_SUPPORT_KTX2_ZSTD to 1

would then suffice ?

Would else would need to be considered ?

[andreasplesch]

With these options, eg. only ASTC support the wasm shrinks to 260KB which is bit better.
With BASISD_SUPPORT_ASTC_HIGHER_OPAQUE_QUALITY it goes up to 330KB.
Let's see if this is useful.