manually revert change to canvas expose handling in fee026c5ef7107d5d594159f5ece5917041591f7

Breaking out the rectangles implies a z-axis ordering of drawing,because the rect drawn last will implicitly be "on top".
But redraw areas are not submitted with any z-axis information, and so drawing like this breaks canvas layering. It
would be more efficient to draw the rects separately, but we don't have any ordering information and so we cannot do it
correctly.