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/wrap_Graphics.h

@@ -42,6 +42,7 @@ namespace opengl
 
 int w_reset(lua_State *L);
 int w_clear(lua_State *L);
+int w_discard(lua_State *L);
 int w_present(lua_State *L);
 int w_isCreated(lua_State *L);
 int w_isActive(lua_State *L);