Seems the call to disable drop shadow on Mac is working after all. It might be a bug in Gtk or my usage of Gtk that is generating the error from elsewhere.
Removed no longer required check for SWF in GtkSpriteInterface.
Changed alpha blending to use fixed point arithmetic when rescaling since this is much faster than the float to int conversion. The rescaling methods are now fairly close in performance to the native size methods.

Extracted a couple of common methods for initialization outside of the loops.