[quartz] Cleanup and make better use of cairo_quartz_image_surface_t.

Use a CGBitmapContext mapping the underlying image surface's data instead of maintaining a CGImage. Generalize the quartz surface snapshot mechanism to work with both cairo_quartz_surface_t and cairo_quartz_image_surface_t and to use the latter to get a CGContext around non-quartz surfaces. Use this snapshot machanism to get a CGImageRef when needed from a cairo_quartz_image_surface_t.