2003-07-16  Michael Natterer  <mitch@gimp.org>

	* app/paint/gimppaintoptions.[ch]: added utility function
	gimp_paint_options_get_fade() which calculates an opacity
	value from paint_core->pixel_dist.

	* app/paint/gimppaintbrush.c: removed the same code here and use
	gimp_paint_options_get_fade().

	* app/paint/gimpclone.c
	* app/paint/gimpconvolve.c
	* app/paint/gimpdodgeburn.c
	* app/paint/gimperaser.c
	* app/paint/gimpsmudge.c: enabled fade for all paint tools, along
	with a general opacity cleanup:

	Use the opacity from gimp_context_get_opacity() *only* for the
	image_opacity. In particular, *never* use it as initial value for
	calculating the brush_opacity. Instead, start calculating the
	brush_opacity from gimp_paint_options_get_fade() and return early
	if it returns 0.0, if not, multiply tool specific opacity sources
	like the current pressure.

	(This changes the effect of the paint tools for particular opacity
	values, but makes the impact of opacity on the final rendering
	linear and more intuitive)

	* app/tools/gimppaintoptions-gui.c: enabled the "Fade" frame for
	the tools above.

	* app/paint/gimppaintcore.c: purely cosmetic cleanup.