Videoplayer frame dropping support

Second try.

Added frame dropping support, i.e. video frames will be skipped when the video clock is running behind the master clock. I tested this by adding a sleep() in VideoState::update.

The comment suggested to implement syncing in the decoding stage, but I see little to no benefit from that. Video formats tend to have dependent frames, so there's a high chance ffmpeg would have to internally decode the skipped frames anyway.

@KCat: could you review this, please?