Add ass_set_pixel_aspect(), deprecate ass_set_aspect_ratio()
ass_set_aspect_ratio() is confusing, because it takes a DAR and SAR
value, while libass just needs a single pixel aspect ratio. Introduce
ass_set_pixel_aspect(), which sets the pixel aspect ratio directly.
ass_set_aspect_ratio() is considered deprecated. There's no reason to
remove it, but hopefully directing users to ass_set_pixel_aspect() will
make for a simpler API.
Improve the doxygen and document what ass_set_margins() actually does.