Before, drawtext filter deliberately altered given text coordinates if
text didn't fully fit on the picture. This breaks the use case of
scrolling large text, e.g. movie closing credits.
Add 'fix_bounds', to make it usable in such cases (by setting its value to 0).
Default behavior is not changed, and non-fitting text coords are fixed.

Signed-off-by: Anton Khirnov <anton@khirnov.net>