Mostly pedantic bikeshedding issues.
Move some code around, so that the sub_bitmap_to_mp_images() function
can be split into two parts. This is better than having a big function
with many input and outputs, of which only half are used in each code
path.
Also, try to make code simpler by using a mp_rect type.
Most of these cased working when the OSD was switched to libass, or
didn't do anything even before that.
Also don't recursively include subreader.h in sub.h.
libswscale tends to overwrite the area between (w,y)-(0,y+1). It tries
to process multiple pixels at once, and if the memory past the last x
pixel is inside a SIMD operation, but still below the image stride, it
overwrites that data with black.
This happens with vo_x11 and 32 bit RGBA formats. The bug is visible as
black bar right of the subtitle bounding box. Fix by giving libswscale
more alignment. Then the "outside" pixels are inside, and are processed
normally instead of overwritten with black.
NOTE: we do not increase the alignment constant, because this is a
separate issue from pointer alignment. libavutil's av_malloc() wouldn't
actually satisfy the increased alignment either.