Cover the frames that aren't three YUV planes

[NicolasHug]

test_planes_structure asserted that a frame has exactly three planes, which
frame_planes() never promised: it hands out one view per component, so a
grayscale frame has one and an alpha one has four. Add a source for each of
the three shapes that were untested - monochrome, planar RGB and alpha - and
assert the count the case expects.

They pin down two things worth pinning: planar RGB comes out (R, G, B), not
in gbrp's own green-blue-red order, and an alpha plane is full size rather
than subsampled like chroma.

Also range-check the NVDEC pitch before narrowing it to an AVFrame line
size, and rename StandAloneFrameAttachedData to OwnedFrameStorage.

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