One sub_bitmaps struct could contain either a libass ASS_Image list, or
a mplayer native list of sub-bitmaps. This caused code duplication in
vo_vdpau.c and bitmap_packer.c.
Avoid this by creating such a sub_bitmap array even with libass. This
basically copies the list and recreates it in mplayer's native format.
It gets rid of the code duplication, and will make implementing extended
subtitle and OSD rendering in other VOs easier.
Also do some cosmetic changes and other preparations for the following
commits.
