[Canvas] Use of PixelWriter to write single pixels leaves the graphics in "SRC" mode

The code to handle single pixel writes in the PixelWriter on a GraphicsContext accidentally leaves the internal graphics object in a SRC composite mode for subsequent rendering even if that rendering is supposed to be SRC_OVER.

The attached test case should draw 2 identical diagrams. The figure on the right blends the blue over the red, but in the one on the right the blue rectangle overwrites the red one.