Screen sharing demo project

It might be nice to move the create into `initCapture` and the cancel/release into `releaseCapture`, however I vaguely remember seeing a crash when I released a dispatch source that was suspended. Would have to check that it's actually safe to do it that way.

The other approach is to do what we did with the camera - start/resume the dispatch source during `initCapture` and just use the _capturing flag to dictate behavior. Only problem there is that stopCapture can return while a frame is still being consumed. Again, you'd have to check to see if it's safe.