wayland: Ensure pointer constraint region consistency

Changes in games between fullscreen and windowed modes may trigger chaotic situations where the buffer and the frame size temporarily disagree, producing rectangles with negative width/height. This is usually followed by other updates that bring the pointer constraint up to date.

This makes cairo panic and return an "error" empty region, which breaks deeper down when using the region rectangles to apply the pointer constraint.

If we hit this situation, ignore the frame rectangle, and just go with the buffer rectangle.