Skip useless memset() when copying a bitmap
shaper: add FriBidi error handling

It is unlikely, but FriBidi might not process a string correctly,
and we should handle that. Tested by making it fail always. This
should also fix some compiler warnings.
Merge pull request #102 from grigorig/fribidi-errorhandle

shaper: add FriBidi error handling
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