Again worthless, slow, and only for libswscale parity.
With this, we support all formats libswscale supports, except bayer
input, and rgb4/bgr4 output. We even support some formats libswscale
doesn't.
It's possible that the zimg wrapper isn't always as fast as libswscale.
But there is optimization potential: the inner repack loops are
self-contained enough that they could be reasonably be implemented in
assembler (probably), and doing everything slice-wise should reduce the
overhead of the separate pack/unpack stages.
wm4
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