Move sub-bitmap definitions from dec_sub.h to sub.h. While it's a bit
odd that OSD data structures are in a file named sub.h, it's definitely
way too strange to have them in a file about subtitle decoding. (Maybe
sub.h/.c and the sub/ directory should be split out and renamed "osd"
at a later point.)
Remove including ass_mp.h (and the libass headers) where possible.
Remove typedefs for mp_eosd_res and sub_bitmaps structs.
Store a mp_eosd_res struct in osd_state instead of just w/h. Note that
sbtitles might be rendered using different sizes/margins when filters
are involved (the subtitle renderer is not supposed to use the OSD res
directly, and the "dim" member removed in the previous commit is
something different).
