mlv_play fix for "exact" playback mode
Background: last night I've recorded a full-res silent timelapse, and had some difficulties playing it back on the camera. Attempting to play it back frame by frame would have taken forever, so I enabled the "exact" mode. It attempted to play back at 1 frame per minute, so I forced the frame rate to 24 fps for very slow videos (timelapses).
Surprise - even in "exact" playback mode, it was still playing frame by frame (it was supposed to skip a lot of frames).
After this test, I realized the "exact" playback mode never worked for MLV files if it had to skip frames (but it probably worked for RAW files - didn't try; only looked at the code). This PR should fix the issue.
I've also cleaned up the FPS timer code a bit.