KTX2Loader: Improve transcoder target format selection

[donmccurdy]

Description

Related:

• KTX2Loader: Support transcoding UASTC HDR to BC6H and RGBA16 #29730

As described in the issue above, Firefox may indicate support for compressed formats that it actually emulates in drivers, decompressing or transcoding to another format. This emulation has much higher cost than just transcoding to the 'right' format in KTX2Loader, but we don't necessarily know what the right format is because Firefox doesn't say. We don't have this problem on ANGLE, but currently we are preserving a less-optimal choice of target transcoding format for ANGLE because of the Firefox issue.

Solution

• Consider detecting the Firefox user agent and adjusting format selection as a special case, if we can do so reliably
  • For Linux+Firefox+RadeonSI we should prefer BCn formats over ETC2 as the target from ETC1S, when both are "supported"
  • For other platforms, is the reverse happening? Would Firefox report BCn support when only ETC2 is actually available?
• Test major browsers on multiple platforms, check target format selection, and implement unit tests for transcoder target formats if possible (WASM in tests may be tricky...)
• Consider preferring BC1/3 over BC7 for ETC1S transcoding
  • See ETC1S desktop decoding guidelines may be inefficient KhronosGroup/3D-Formats-Guidelines#25
• File an issue with Firefox about the emulated support
• Fix invalid WebGPU feature detection in KTX2Loader for (non-existent) texture-compression-bptc and texture-compression-etc1

Alternatives

n/a

Additional context

No response