pub fn decompress_chunk(vxl: &Vxl) -> ChunkUploadExpand description
Decompress a Vxl chunk into the GPU upload shape, building the
full mip ladder (gpu_mip_count(vsid) levels). Caller
guarantees vxl is shaped as a roxlap-scene chunk (vsid
square). If vxl already carries at least that many mips (the
common scene path — the bake generates 6), they are read
directly; otherwise the chunk is cloned and re-mipped so the
upload always carries a deterministic, vsid-uniform level count.
mips[0] is the legacy mip-0 data, also mirrored into the
top-level ChunkUpload fields for the older single-chunk /
single-grid paths.