Added test case for PCX.
did some cleanup:
	* image_load and load_targa have been renamed to img_load and img_load_targa, respectively
	* the "rgb checker pattern" tests have been renamed to "rgb_triplet_pattern" to refelect
		the fact that they checkerboard only have two colors.
	* Also, all test images are now placed in the test_data directory after a build to reduce
		the likelihood of a name collision (because putting them in folders named tga and
		pcx was kinda ambiguous).  The old pathnames are still used in test.c; run_test_case
		will automatically add test_data to the beginning.