avcodec/ass,webvttdec: implement more portable curly brace escapes

Unlike what the old comment suggested, standard ASS has no character
escape mechanism, but a closing curly bracket doesn't even need one.

For manual authored sub files using a full-width variant of an
appropriate font and with scaling and spacing modifiers is a common
workaround.
This is not an option here, but we can still make things much less bad.
Now the desired opening bracket still shows up in libass, and
standard renders will merely display a backslash in its place
instead of stripping the following text like before.

0 files changed, 0 insertions, 0 deletions