Optimize area screen casting in single-monitor case

The largest part of computer users uses only a single monitor, in this case we can avoid painting the stage a second time in the area stream and simply use the monitors framebuffer as the source of our recording.

So introduce an optimization for the area stream that compares our rectangle to the logical monitor rects that exist, and make use of the functions to record monitor buffers/framebuffers that we factored out of the monitor stream source in the last commits.

Depends on !1701 (merged) (just because I had it applied anyway)