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.
In order to improve performance, vo_xv didn't create a backup of the
video frame before drawing OSD and subtitles during normal playback. It
required the frontend to do frame stepping if it wanted to redraw the
OSD, but no backup of the video frame was available. (Consider the
following use case: enable the OSD permanently with --osd-level=3, then
pause during playback and do something that shows an OSD message. The
player will advance the video by one frame at the time the new OSD
message is first drawn.)
This also meant that taking a screenshot during playback with vo_xv
would include OSD and subtitles in the resulting image.
Fix this by always creating a backup before drawing OSD or subtitles.
In order to avoid having to create a full copy of the whole image frame,
introduce a complex scheme that tries to backup only the changed
regions.
It's unclear whether the additional complexity in draw_bmp.c for
backing up only the changed areas of the frame is worth it. Possibly
a simpler implementation would suffice, such as tracking only Y ranges
of changed image data, or even just copying the full frame.
vo_xv's get_screenshot() now always creates a copy in order not to
modify the currently displayed frame.
