Fix dynamictext #timecode# with 59.94 fps.

Fixed by refactoring to use mlt_properties_frames_to_time().

Reported-by: Christophe Thommeret
Merge branch 'master' of github.com:mltframework/mlt
Refactor data_show #timecode# to use mlt_properties_frames_to_time().

Also, a couple of fixes to prevent string overflow.
Fix a memory leak encoding on FFmpeg v2.3.
Fix corrupt image attempting to pad planar image.
Let mlt_image_format property choose SDL overlay mode.