The mplayer DVD sub decoder is the only remaining OSD image producer
that still requires the old mplayer OSD format (SUBBITMAP_OLD_PLANAR).
To make supporting this format optional in VOs, add a step that allows
converting these images to RGBA in case the VO doesn't have direct
support for it.
Note: the mplayer DVD sub decoder uses the old mplayer OSD format
(SUBBITMAP_OLD_PLANAR), which is assumed to use premultiplied alpha.
However, it seems DVDs allow only binary transparency, so the rendered
result will be the same.
Before this commit, the OSD was drawn using libass, but the resulting
bitmaps were converted to the internal mplayer OSD format. We want to
get rid of the old OSD format, because it's monochrome, and can't even
be rendered directly using modern video output methods (like with
OpenGL/Direct3D/VDPAU).
Change it so that VOs can get the ASS images directly, without
additional conversions. (This also has the consequence that the OSD can
render colors now.) Currently, this is vo_gl3 only. The other VOs still
use the old method. Also, the old OSD format is still used for all VOs
with DVD subtitles (spudec).
Rewrite sub.c. Remove all the awkward flags and bounding boxes and
change detection things. It turns out that much of that isn't needed.
Move code related to converting subtitle images to img_convert.c. (It
has to be noted that all of these conversions were already done before
in some places, and that the new code actually makes less use of them.)
