Added love.graphics.clearStencil, which only clears the stencil buffer.

--HG--
branch : minor

src/modules/graphics/opengl/Graphics.h

@@ -138,6 +138,11 @@ public:
 	void setStencilTest(bool enable, bool invert);
 	void getStencilTest(bool &enable, bool &invert);
 
+	/**
+	 * Clear the stencil buffer in the active Canvas(es.)
+	 **/
+	void clearStencil();
+
 	/**
 	 * Creates an Image object with padding and/or optimization.
 	 **/