Fixed angle_end/start with new circle drawing algorithm

The problem related to the old (slower) algorithm using the absolute value of the angle change, whereas the new one depends on the sign.

~~This still doesn't fix a separate bug where if the width/height ratio isn't 1, the texture can be cropped (taking a circle out of the image rather than showing the full ellipse), so the old algorithm is still used if width != height.~~

Edit: I've now also fixed the texture crop bug, so the new algorithm gives (as far as I can tell) exactly the same results as the old one. I haven't benchmarked it though...not sure the best way.

This PR also updates the circle example to allow control of stretching.