Added 'drawcallsbatched' field to the table returned by love.graphics.getStats.

It has the number of draw calls that were saved by auto-batching.

--HG--
branch : minor

src/modules/graphics/opengl/Graphics.cpp

@@ -1010,6 +1010,7 @@ void Graphics::present(void *screenshotCallbackData)
 	gl.stats.drawCalls = 0;
 	gl.stats.shaderSwitches = 0;
 	canvasSwitchCount = 0;
+	drawCallsBatched = 0;
 }
 
 void Graphics::setScissor(const Rect &rect)