sub: change how libass output is converted to RGBA in some cases

This affects VOs (or other code which render OSD) which does not support
the LIBASS format, but only RGBA. Instead of having a converter stage in
osd.c, make mp_ass_packer_pack() output directly in RGBA.

In general, this is work towards refcounted subtitle images.

Although we could keep the "converter" design, doing it this way seems
simpler, at least considering the current situation with only 2 OSD
formats. It also prevents copying & packing the data twice, which will
lead to better performance. (Although I guess this case is not important
at all.)

It also fixes --force-rgba-osd-rendering when used with vo_opengl,
vo_vdpau, and vo_direct3d.

sub/osd_state.h

@@ -5,8 +5,6 @@
 
 #include "osd.h"
 
-#define OSD_CONV_CACHE_MAX 4
-
 enum mp_osdtype {
     OSDTYPE_SUB,
     OSDTYPE_SUB2, // IDs must be numerically successive
@@ -48,10 +46,6 @@ struct osd_object {
     // OSDTYPE_EXTERNAL2
     struct sub_bitmaps *external2;
 
-    // caches for OSD conversion (internal to render_object())
-    struct osd_conv_cache *cache[OSD_CONV_CACHE_MAX];
-    struct sub_bitmaps cached;
-
     // VO cache state
     int vo_change_id;
     struct mp_osd_res vo_res;