Improve Fast-forwarding

[aidancrowther]

Fast-forwarding currently operates by disengaging the frame limiter to allow the renderer to pull frames as they are decoded. This allows the video to run at a faster pace, however, it is quite slow once the renderer catches up to the buffer head.

Instead, the buffer should be caught up to and cleared as necessary, and then frames should be skipped in the loader to increase the rendering pace, going back to normal once the fast forward is terminated.