ClearRectangle now erases rectangular regions on Canvas and Picture graphics

Graphics.ClearRectangle(x, y, width, height) did not visibly clear anything. After filling a Canvas red and clearing an inset rectangle, the entire surface stayed solid red instead of leaving a red border with the centre cleared out.

The clear now erases the rectangular region as documented: drawing drawn earlier in that region is removed, revealing the canvas background behind it (or the canvas Backdrop image when one is set).