Redefine RGB_t as unsigned char.

src/graphics/graphics.h
	-Redefine `RGB_t` as an `unsigned char`, since it only
	constitutes a single third of any color and is capped at 0xFF.

src/unit_tests.c
	-Redefine the unit-test constant in `testLighting()`, because the
	original didn't seem to account for specular light.