KTX2Loader: Add `VK_FORMAT_ASTC_6x6_SFLOAT_BLOCK_EXT`

[hybridherbst]

Description

I created some VK_FORMAT_ASTC_6x6_SFLOAT_BLOCK_EXT files for testing and noted that three.js currently can't load those.
VK_FORMAT_ASTC_4x4_SFLOAT_BLOCK_EXT works fine.

I think KTX-Parse supports it already:
https://github.com/donmccurdy/KTX-Parse/blob/634671efca481038ad3c4b01ac51e4af5a14fc54/src/constants.ts#L323

Attached are 4 files – 4x4 with/without mips and 6x6 with/without mips:
ASTC_BLOCK_SFLOAT.zip

Solution

Support for the format

Alternatives

Using 4x4 is an alternative to 6x6, but files are ~2x larger.