Support showing video transparency in the editor #237

Currently, the video player in the editor just shows a black background for videos with an alpha channel. I think this is a limitation in AVPlayerView, but if anyone knows a way to make it work, let me know. According to this session from WWDC 2019, AVPlayer should support videos with alpha channel.

<img width="917" alt="Screen Shot 2021-03-15 at 00 30 18" src="https://user-images.githubusercontent.com/170270/111077924-a137b980-8525-11eb-81fc-a255f584546b.png">

Here's a sample video: puppets_with_alpha_hevc.mov.zip

Feedback Assistant report: https://github.com/feedback-assistant/reports/issues/197