drawtext: add 'fix_bounds' option on coords fixing

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>

doc/filters.texi

@@ -598,6 +598,9 @@ libfreetype flags.
 @item tabsize
 The size in number of spaces to use for rendering the tab.
 Default value is 4.
+
+@item fix_bounds
+If true, check and fix text coords to avoid clipping.
 @end table
 
 For example the command: