Change Graphics::reset() to unbind FBO and clear stencil buffer. Change error

screen to call love.graphics.reset() instead of individual resets.

--HG--
branch : minor

src/modules/graphics/opengl/Graphics.cpp

@@ -297,6 +297,8 @@ namespace opengl
 	void Graphics::reset()
 	{
 		DisplayState s;
+		discardMask();
+		Framebuffer::bindDefaultBuffer();
 		restoreState(s);
 	}