Don't show a negative size canvas.

This is in line with how Rectangle and Image elements
also behave.

Change-Id: I82cd6ecc6d233645bb3fd968f0f81218188a8991
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>
Our Context2D::clearRect implementation was a bit clumsy

Change-Id: I273dd5eb73dce07a9ca0a1ac72a0780b0393603a
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>