Add partly functional Goraud shading.

Add:
	src/graphics/graphics.c
		-Add `INTERPOLATE_COLOR()` macro, used throughout Goraud
		shading; implement the Goraud algorithm fully in `flatShade()`,
		which now calls `drawHorizontalGradientLine()` with interpolated
		endpoint lights.
Merge branch 'flat_shading' of github.com:sevko/graphics-engine into flat_shading

Conflicts:
	src/graphics/graphics.c

Reverted the engine to c13967e and forgot to push before committing 8a80985.