ffmpeg: Add threshold to discard crazy/damaged timestamps.

The added tests are limited to the case where timestamp discontinuities
are not allowed. The default is 30 hours which is arbitrarily picked and
quite conservative.
This prevents a out of memory condition due to duplicating a frame
millions of times.

Found-by:  Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>