Add low-level hardware instancing support to Meshes.

Add “instancing” Graphics Feature constant.

Add love.graphics.drawInstanced(mesh, instancecount, x, y, …) which draws the mesh multiple times in a single draw call. Each instance of the mesh will appear in the exact same spot unless one of the following are used:

- Add an optional vertex attribute step type argument to Mesh:attachAttribute. “pervertex” is the default. “perinstance” causes the attribute to be per-instance instead of per-vertex, when the mesh is drawn.

- Add love_InstanceID as a built-in read only int variable in GLSL 3 vertex shaders. It is always 0 except when a Mesh is drawn with more than 1 instance specified in the draw call. It can be used to manually compute or index into per-instance values in a shader.

--HG--
branch : minor

src/modules/graphics/opengl/Graphics.h

@@ -96,6 +96,8 @@ public:
 
 	void flushStreamDraws() override;
 
+	void drawInstanced(Mesh *mesh, const Matrix4 &m, int instancecount);
+
 	void clear(Colorf color) override;
 	void clear(const std::vector<OptionalColorf> &colors) override;