Blend images in Context2D PDF exporter with brush.

Simply dropping the alpha channel created unexpected behavior when some images in a scene were had transparent black (0, 0, 0, 0) backgrounds and others had transparent white (1, 1, 1, 0) backgrounds.

Now, rather than simply dropping alpha, the image is flattened into a background consisting of the brush's color. This allows the drawer to have a bit more control over these situations.