* app/paint_funcs.c, app/transform_core.c:  replace the cubic
	functions with a better/faster version supplied by
	David Hodson <hodsond@acm.org>

	* app/brush_select.c: double clicking on a brush preview now opens up
	the brush editor.  Update the text displayed when the active brush
	changes size.

	* app/paint_funcs.c: slight modification to overlay_pixels.