OpenGL 2.1 is now required to use love.graphics (resolves issue #779.)

Removed love.graphics.isSupported("canvas", "shader", "npot", "subtractive", and "mipmap"), since those features are all guaranteed by the minimum system requirements.

--HG--
branch : minor

src/modules/graphics/opengl/Graphics.h

@@ -473,6 +473,8 @@ private:
 
 	DisplayState savedState;
 
+	bool displayedMinReqWarning;
+
 }; // Graphics
 
 } // opengl