

The problem with rendered images is that the left side triangle isn’t smooth enough. There is a bit of jagged edge on the slopes. This was found in the draw9patch tool’s preview too, but couldn’t tackle it so far.
Any hint regarding solution will appreciated.
This happens because you specify too wide area for the height shrink/stretch. Put just one black pixel in the leftmost pixel column (it’s tricky where to put it, actually. I’d put one pixel in the very top of your image and one more – in the very bottom).