If --blend-subtitles=yes is given, vo_opengl will call osd_draw()
multiple times, once for subtitles, and once for OSD. This meant that
the want_redraw flag was reset before the OSD was rendered, which in
turn meant that update_osd() was never called. It seems like removing
the per-OSD object want_redraw wasn't such a good idea. Fix it by
reintroducing such a flag for OSDTYPE_OSD only.
Also, the want_redraw flag is now unused, so kill it.
Another regression caused by commit 9c9cf125. Fixes#3535.