* WIP compressed textures, swizzles, sRGB sampling, ...
* refactor texture type info & fix random bugs
* fix preprocessing of sRGB textures
* handle y-flipped basis textures
* glcommon: better WebGL compat for compressed format detection
* missed WEBGL_compressed_texture_pvrtc
* implement compressed texture xcoding and uploading
* Add basis_universal submodule
* Reorganize texture loader code
Clean up some code
Isolate Basis Universal loader into a separate module
* Add wrapper script for encoding .basis textures
* basisu: honor custom metadata written by the mkbasis.py script
* mkbasis.py: add --incredibly-slow and --dry-run
* Move pixmap code from util/ to pixmap/
* Add an on-disk transcode cache for basis textures to speed up loads
* Compress texture cache with zlib
* Use readable format names for basisu cache filenames
* basisu: mip bias test code
* basisu: small caching cleanup
* add TAISEI_BASISU_MIP_BIAS env variable
* Improve OpenGL format matching heuristics
* Document considerations for compressed format priority
* Remove dead code
* Enable two forgotten formats, BC3_RGBA and ATC_RGBA
Also prefer BC7 over BC1/BC3
* Recognize GL_ANGLE_compressed_texture_etc for ETC2 textures
* Default depth buffers to 24-bit; remove ANGLE hack
* Fix glcommon_check_extension for GLES2/legacy gl
* Add renderer feature bit for texture swizzle masks
* glcommon: Fixup internal formats for GLES2
Sized internal formats are not allowed in GLES2
* Fix emscripten compile errors
* Update basis_universal
* remove more dead code
* revert irrelevant stage4 change
* shut up UBSan
* basisu: shut up some debug spam
* Add normalmap sampling helper to util.glslh
* basisu: add a gray-alpha mode
* mkbasis.py: Abort if image dimansions aren't multiples of 4
* Add basic Basis Universal encoding documentation (WIP)
* doc/basisu: Add paragraph about modes; minor tweaks
* basisu: workarounds for GL texture size requirements
* gles20: fix uncompressed sRGB formats
* Partial workaround for missing swizzles in gles2 and webgl
* remove invalid assertion
* New renderer API to expose glDrawBuffers-like functionality
* stagedraw: disable all color outputs for copy_depth pass
required for WebGL compatibility
* support GL_ANGLE_request_extension
* emscripten: include *.basis in gfx package
Also fix a potential problem when more than one .pkgdir is used to
construct emscripten packages
* Don't rely on emscripten runtime to enable webgl extensions
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