In order to support OSD redrawing for vo_xv and vo_x11, draw_bmp.c
included an awkward "backup" mechanism to copy and restore image
regions that have been changed by OSD/subtitles.
Replace this by a much simpler mechanism: keep a reference to the
original image, and use that to restore the Xv/X framebuffers.
In the worst case, this may increase cache pressure and memory usage,
even if no OSD or subtitles are rendered. In practice, it seems to be
always faster.
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