parse_tag: split \fscx, \fscy, \fsc
parse_tag: merge \fs+, \fs-, \fs
Move (r)skip_spaces to ass_utils
Parse override tag arguments exactly like VSFilter 2.38

Also replace strtocolor in ass_utils with string2color
from ass.c, because that is more useful everywhere now.
Introduce ass_drawing_add_chars for adding a whole string at once
Require closing '}' for override tags

Like VSFilter. '{' without a following '}' is just text, though
in vector drawing mode it still delimits individual drawings.

This also lets us nicely avoid '\0' hacks
in the \t override tag handler in parse_tag.
parse_tag: remove unnecessary malloc
Use ass_drawing_add_chars for vector drawings

This not only provides a performance improvement but also
conveniently fixes the following issue: we used to interpret
backslash-escapes in vector drawings, but we shouldn't.
Simplify drawing text assignment