videoconvertscale: ensure writable caps when fixating format
gst_video_convert_scale_get_fixed_format() receives 'othercaps' from
basetransforms' fixate_caps() vmethod which explicitly mentions that
'othercaps
may not be writable'.
The gst_caps_intersect() call just before may or may not produce new caps. Particularly in cases like EMPTY or ANY caps on either of the inputs, only a ref is taken and returned to the caller.
As a result, gst_video_convert_scale_fixate_format() may have attempted to modify a non-writable caps structure.
Fix by adding a gst_caps_make_writable().