Instead, sd_lavc.c and spudec.c (the two image sub decoders) always
output indexed/paletted images. For this purpose, add SUBBITMAP_INDEXED,
and convert the subs to RGBA in img_convert.c instead. If a VO is used
that supports the old OSD format only, the indexed bitmaps are converted
to the old OSD format by abusing spudec.c in a similar way sd_lavc.c
used to do.
The main reason why spudec.c is used is because the images must not only
be converted to the old format, but also properly scaled, cropped, and
aligned (the asm code in libvo/osd.c requires this alignment).
Remove support for the old format (packed variant) from the OpenGL VOs.
(The packed formats were how the actual OSD format was handled in some
GPU-driven VOs for a while.)
Remove all conversions from old to new formats. Now all subtitle
decoders and OSD renderers produce the new formats only.
Add an evil hack to convert the new format (scaled+indexed bitmaps) to
the old format. It creates a new spudec instance to convert images to
grayscale and to scale them. This is temporary for VOs which don't
support new OSD formats yet (vo_xv, vo_x11, vo_lavc).
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