Use getNextPts == 0 instead of empty() to tell if we are finished
displaying AVPictureBuffer frames.

In SDL2 we are keeping the existing frame up until it is the next
frame's turn so the AVPictureBuffer is never empty unless it is
freshly created or deallocated. If there is no next frame
getNextPts is 0.
Move unload call from onMovieOver to _destroyMovie

Calling unload causes the AVPictureBuffer to be flushed, but in
SDL2 we need to keep it so we can keep showing the last frame as
long as the last frame is visible.
Merge pull request #125 from TheCycoONE/sdl_movie_fix

SDL2 movie fix for #120