Previously the playback mode "Frame Dropping" would not drop the correct
number of frames which would lead to slow playback.
For example, the playback target is 60fps. However we can only muster
around 32 fps.
The delta frames from the last step is in this case ~1.98 or so.
With the previous code, we would floor this. That would lead us to step
forward one frame each time, effectively playing back the animation at
half the speed as we will try to render every frame.
To fix this we simply save the remaining fraction from the previous
frame and use it to compute the current frame step.
Reviewed By: Sybren
Differential Revision: http://developer.blender.org/D7694
10528a5cd4