Until now, subtitle renderers could export SUBBITMAP_INDEXED, which is a
8 bit per pixel with palette format. sd_lavc.c was the only renderer
doing this, and the result was converted to RGBA in every use-case
(except maybe when the subtitles were hidden.)
Change it so that sd_lavc.c converts to RGBA on its own. This simplifies
everything a bit, and the palette handling can be removed from the
common code.
This is also preparation for making subtitle images refcounted. The
"caching" in img_convert.c is a PITA in this respect, and needs to be
redone. So getting rid of some img_convert.c code is a positive side-
effect. Also related to refcounted subtitles is packing them into a
single mp_image. Fewer objects to refcount is easier, and for the libass
format the same will be done. The plan is to remove manual packing from
the VOs which need single images entirely.
f72eb5b394