Provide an attempted optimization when damage reports appear wrong.

With compositing window managers like mutter, when used with the spice-video-dummy, you can get damage reports that are the whole screen.

Essentially, you just get an indication that the screen has changed, but no sense of what has changed.

This change detects that behavior and stops trusting those damage reports, and instead increases the scan frequency to compensate.

v2: A small style change and a simplification of the detection algorithm.