CairoCanvas: added DrawSurface method
CairoExport: fix backend selection
CairoExport: fix filling frame with solid color, should be transparent.
CairoExport: paint image background except if it is stretched and not keepaspect
because in such case the image covers the full area
CairoExport: fix images, implement shared images (PDF backend)
CairoExport: clip unstretched images