Capture and show screenshots of the program being recorded

[burg]

[burg]

Some major hurdles here:

1. "camera" probe implementation in backend that uses Frame::imageFromRect() and friends to make the screenshots at a suitable size, resolution, etc.
2. Encoding the data in base64, sending to Inspector frontend, and decoding into image.
3. UX to show images on timeline, etc. unobtrusively without blowing up performance.