Calculation of frame count disagrees with camtasia

[abingham]

We calculate the frame counts of videos using pymediainfo. Generally speaking this seems to give good results, but occasionally we get results which different from camtasia. In particular, we sometimes calculate one more frame than camtasia. When we use this frame count to extend the last frame of a video into a stitched media, this results in camtasia actually displaying a black frame (i.e. because it doesn't believe the frame we ask to extend actually exists).