Implemented fading dynamics calculation.
Let fade work with all driven factors. Added pixel_dist parameter to get_dynamic_x.
Adjusted function calls for get_dynamic_x.

        * app/paint/gimppaintoptions.c: Added initialization for fading dynamics.
: Added fading related variables similar to Random.
: Adjusted get_dynamics_mix function.
: Added fading calculation in get_dynamics_x functions.
: Adjusted get_dynamic_x functions for fade dynamics.

	* app/paint/gimppaintoptions.h: Added fading_options in GimpDynamicOptions.

	* app/tools/gimppaintoptions-gui.c: Added the function fading_options_gui.

	* app/core/core-types.h: Added fading as the extra point in Gimpcoords.
(not quite sure if it is correct.)

	* app/paint/gimpclone.c
	* app/paint/gimpdodgeburn.c
	* app/paint/gimperaser.c
	* app/paint/gimpheal.c
	* app/paint/gimppaintbrush.c
	* app/paint/gimpsmudge.c
	* app/paint/gimpairbrush.c
	* app/paint/gimpbrushcore.c
	* app/paint/gimpconvolve.c:Adjusted function calls.
get_dynamic_opacity function calls were adjusted
to have one more parameter, paint_core->pixel_dist.

some key words:
get_dynamic_
pixel_dist
paint_core->pixel_dist