Done to keep ABI compatible. Otherwise this is just silly
Compute the outpicref PTS in start_frame(), since it is required for comparing that value with the value of the overpicref. Originally committed as revision 25828 to svn://svn.ffmpeg.org/ffmpeg/trunk
adopting Baptiste variant which is simpler and faster. Originally committed as revision 25784 to svn://svn.ffmpeg.org/ffmpeg/trunk