Add (low level) functionality to allow rendering 3D Meshes.

- Add love.graphics.setDepthMode(testcomparison, enablewrites).
- Add love.graphics.setMeshCullMode.
- Add love.graphics.setFrontFaceWinding.

Depth testing and depth writes require a depth buffer to work. The mesh cull mode controls whether front/back faces of a mesh are culled. The front face winding determines whether a clockwise or counterclockwise-oriented face in a mesh is considered front facing.

--HG--
branch : minor

src/modules/graphics/depthstencil.h

@@ -48,6 +48,7 @@ enum CompareMode
 	COMPARE_GREATER,
 	COMPARE_NOTEQUAL,
 	COMPARE_ALWAYS,
+	COMPARE_NEVER,
 	COMPARE_MAX_ENUM
 };