Add light unit-test; exit with appropriate status.

Add:
	src/unit_tests.(c, h)
		-Add `testLighting()` unit-test for `flatShade()`.
		-Modify `TEST()` and redefine `unitTests()` to return the
		unit-tests' exit-status.
Rename flatShade() to lightColor().

Add:
	src/graphics/graphics.(c, h)
		-Rename `flatShade()` to `lightColor()`, which more accurately
		conveys the function's role (it only applies a series of
		equations and has nothing to do with the surface colors that
		flat-, gouraud-, etc. shading handle).