[what] Fix and finish implementation of Video Transition
[why] Original code is unfinished, and contain memory leak, which cause it can't play over 10 seconds.
[how] 1. revise architecture of Video Transition.
2. Add FrameMixer to mix frames
3. Add Transition effects
[note] some issues TBS: 1. the video can't play second time 2. the "Playback page is disabled"