* app/apptypes.h: added InterpolationType enum.
	* app/gimprc.[ch], app/preferences_dialog.c: replaced
	cubic_interpolation flag with interpolation_type variable.

	* app/pixel_region.[ch]: added pixel_region_has_alpha function.

	* app/paint_funcs.c: rewrote scale_region.  It now behaves
	correctly on images with alpha, no longer leaves an artifact on the
	right edge of images when scailing up, and runs signifigantly
	faster.