Hack: set the coded frame PTS to the incoming PTS.

This is not correct, but libtheora does not seem to provide a way
to get the correct value, and this is necessary to make encoding
produce sensible time stamps when encoded content is variable
FPS or the time base is simply different from FPS.
Somewhat fixes issue 1197.

Originally committed as revision 20199 to svn://svn.ffmpeg.org/ffmpeg/trunk