Implement vsync

[ardera]

Currently, flutter-pi runs without vsync, which results in visible tearing. In the future, there should be vsync support, but I first have to figure out how to do this.
There are multiple ways you can implement vsync:

• you can use the legacy (?) drm way using page-flips
• or use the atomic way, native fence etc.

I have no idea which is better (in terms of performance, compatibility, etc). Compatibility would be nice to have, since this embedder may support other platforms than raspberry pi in the future.

[ardera]

Implemented in #24, although I'm not sure it's fully working.

Tearing is definitely a lot better than before, but it seems sometimes it still occurrs.