DR (direct rendering) works by having the decoder decode into the GPU
staging buffers, instead of copying the video data on texture upload. We
did this even for formats unsupported by the GPU or the renderer. This
"worked" because the staging memory is untyped, and the video frame was
converted by libswscale to a supported format, and then uploaded with a
copy using the normal non-DR texture upload path.
Even though it "works", we don't gain anything from using the staging
buffers for decoding, since we can't use them for upload anyway. Also,
staging memory might be potentially limited (what really happens is up
to the driver). It's easy to avoid, so just skip it in these cases.
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