Fixed an issue with cairo::SurfaceImage's copy constructor...

...as reported by Matija Koželj. See https://forum.libcinder.org/#Topic/23286000002066079 . This fixes issue #452.
This seems like it was missing indeed. Though I don't know the Cairo stuff very well, I'm looking at the SurfaceImage constructor that takes an ImageSourceRef and [it makes a couple other calls](https://github.com/cinder/Cinder/blob/master/blocks/Cairo/src/Cairo.cpp#L227-L228) - should `cairo_surface_mark_dirty( mCairoSurface )` be called in the copy constructor as well?