Video converter: nearest neighbour scaling corruption
Fix corruption when using nearest neighbour scaling that's caused by generating unnecessary intermediate data into a shared temporary pixel buffer that actually points to the output video frame. Also add a compositor example that exercises the problematic case.
video-converter: Skip input lines where possible.
There is a case where there are no lines in the temp cache, and it's possible to skip straight to the request line and not generate intermediate ones. This is really only beneficial when doing nearest-neighbour downscaling, as other methods generally require all input lines sequentially to generate the output. In that case, this change has no effect and all lines are generated and cached as before. As a side effect however, this fixes corruption when downscaling using nearest-neighbour, as interactions with the pass_alloc flag and reuse of temporary lines causes the unecessarily-generated cache lines to overwrite the final output.