Fix bug #366: Cairo render: Group layer doesn't render its context when its content doesn't overlap.
Fix bug #371  -  Checkerboard layer renders darker with alpha < 0 in Cairo
CairoColor: add specialization for Blend mode COMPOSITE
Noise: add new member get_cairocolor to make possible render it in Cairo.
New member get_cairocolor for Metaballs
New member get_cairo_color for NoiseDistort