Playback under MacOSX can fail with window is background

[mdejong]

When a window that is playing ends up in the background, the playback logic will sometime fail and nil is returned for each frame. This can result in the next loop not being preloaded since the current time is not reported properly. Need to figure out how to deal with case of failure to return a frame while still executing next clip load logic even if itemTime is not valid.