Added love.graphics.discard, which discards the contents of the screen (or currently active Canvas.)

Using it just after activating a Canvas can improve performance on many mobile devices, if the conditions for using it are right.

--HG--
branch : minor

src/modules/graphics/opengl/Graphics.h

@@ -85,6 +85,11 @@ public:
 	 **/
 	void clear(Color c);
 
+	/**
+	 * Discards the contents of the screen.
+	 **/
+	void discard(bool color, bool stencil);
+
 	/**
 	 * Flips buffers. (Rendered geometry is presented on screen).
 	 **/