This allows clients to initialize 3D image objects with data in a BC1,
BC2, or BC3 (aka DXTn, aka S3TC) format. Since Metal doesn't support
this natively, DXTn-compressed image data are manually decompressed
prior to upload or copying. This particular algorithm was derived from
Wine, from code originally authored by Connor McAdams. It's somewhat
primitive, and could probably stand some vectorization, but it's a good
first start.
The meat of the algorithm is pulled into a header, "MVKDXTnCodec.def",
intended to be used by both CPU and shader code. CPU uploads, which in
MoltenVK happen when binding a `VkDeviceMemory` object to an image, run
the code normally. GPU uploads, i.e. copying from a buffer to a texture,
run the algorithm as a compute shader, possibly storing the results to
an intermediate buffer to then copy directly to the texture.
Intermediate buffers are used when uploading to mip levels higher than
0, since desktop Metal doesn't support directly writing to higher mip
levels from a shader.
Only uploads are supported. Downloads are not implemented yet.
Theoretically, this could be extended to other compressed formats as
well, allowing MoltenVK to support all texture compression formats on
all platforms.
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