Added some tests of composite to highlight problems in the old

Added some tests of composite to highlight problems in the old

	* gdk-pixbuf/.cvsignore:
	* gdk-pixbuf/Makefile.am:
	* gdk-pixbuf/test-gdk-pixbuf.c (store_pixel), (fill_with_pixel),
	(load_pixel), (simple_composite_test_one),
	(simple_composite_test_one_type), (simple_composite_test), (main):
	Added some tests of composite to highlight problems in the old
	implementation. These tests run without any user interaction.
	Just do "make check".

	* gdk-pixbuf/pixops/pixops.c (pixops_composite_nearest),
	(composite_pixel), (composite_line): Fix composite to do a textbook
	"A over B" composite. This was clearly the intent, and it was easy
	to fix the code to do it. (Note to those that read my earlier tries
	at a patch; this version fixes typos that were present in all
	my patch attempts. I checked the final version by adding more tests.)