Refine Goraud shading; use averaged surface norms.

Add:
	src/graphics/graphics.(c, h)
		-Rewrite and update some documentation to reflect new argument
		names/functionality.
		-Account for the case where `Light_t *` arguments passed to
		`drawHorizontalGradientLine()` are not necessarily in
		left-to-right order along the x-axis.

	src/graphics/matrix.c
		-Use `avgNorms` normals in `drawMatrix()`, instead of the
		surface normal to the triangle (used when testing).