This is extra important in the tests where we expect no changes to the
display, as otherwise we can't tell the difference between success and a
decoder that is simply waiting for more data.
This test didn't really check anything useful as the end result would be
the same if the second JPEG failed to render.
Fix this by clearing the canvas between the images, so we can tell if
the second image actually rendered or not.
Pierre Ossman
pdlan