Overdraw in WorkspaceView::draw_background et al

[Chris Wilson]

For example in WorkspaceView::draw_background, the surface is cleared prior to being painted over by the foreground. The use of CLEAR, OVER is equivalent to a single pass of SOURCE - even if the foreground is semi-opaque. (So long as the second pass does completely fill the surface, which seems to be case.) This idiom is repeated many times.

[Tom Beckmann (tombeckmann)]

Good find, I didn't even realize that before. It might not be applicable for every draw operation but at least for the good majority. In case you would have free time, I'd really appreciate a merge request for this (maybe you'll even notice other places where the drawing could be optimized). Otherwise I'll see when I get time to fix this.