video_format_TransformBy: fix and preserve video offsets
In general, the picture buffer contains, in memory order: - cropped top scan lines, - visible scan lines, - cropped bottom scan lines, - extra codec lines, - padding lines. This order needs to be preserved when flipping vertically or rotating 180 degrees, since filters and displays rely on this. In particular, the picture plane structure so far has no provisions for non-zero offsets, so filters expect visible pixels at the top left of each pixels plane. Effectively, non-zero offsets only work for cropping after filtering. Lets assume that the number of cropped lines are identical at the top and bottom, and keep the X and Y offsets as are. This fixes green lines at the bottom on R180 and VFLIP. If support for non-centered cropping is ever required, I believe offsets will need to be added to plane_t. This would break many video filters though.
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