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.
No, that won't be necessary. The constructors that take a Cinder Surface or an Image source actually construct a new Cairo surface. But in this case, the Cairo surface from the 'other' instance is used directly as the source and is not changed in the process. Therefor we don't need to mark it as 'dirty', as far as I know.