#175 vfr not working properly with MKV files.

[Alexander Wehr]

variable frame rate feature on mkv files not working properly.

specific test files are H.264 video with vfr and aac audio.

H.264 and aac decode.. but at points where framerate deviates the
video begins to chop heavily... stopping and then jerking every
second or so to a point many frames forward.

not good.