copy mCinderSurface ref from other in SurfaceImage copy constructor.

This is an alternative to the cairo::SurfaceImage fix provided in #453, which does not make a deep copy of the ci::Surface but instead copies over a reference to it (because it is still 'implicitly shared').  My understanding is that this is consistent with the existing ci::cairo API and ci::Surface in general to do ref counting on image data.

I've tested that the ref counts go to zero with both methods (this one and @paulhoux's), so its really just a matter of whether the data should be copied over or not.