SDL2 movie fix for #120

Fixes issue #120, caused by the change to the way the current frame is shown in sdl2.  Previously the last frame was left on the screen with no redraw until it was time for the next frame, and the frame pointer advanced immediately.  Now the last frame is redrawn every cycle until it is time for the next frame, and the frame pointer doesn't advance until the next frame's time stamp.

This broke detecting when we were done playing, and freezing on the last frame, which this pull request repairs.