[RFC] get a usable chroma description for opaque chromas
Following !4424 (merged), I made the code more generic by getting the chroma description via the video context.
Even for opaque chromas that match multiple memory layout that should be possible to match to an actual "CPU" chroma description. (the only one that might not work is DXGI_FORMAT_I420OPAQUE
which may not be a real I420 layout).