Fix circle rendering

[smarq8]

very first update 'd' assume 'x=0' and 'y=r' so we can not change them before updating 'd'. I know over internet we can found different versions so I manually checked it on a paper and it seems right.

[tuupola]

Excellent. I did not notice before that circles were slightly malformed. This can be quite clearly seen in the smaller radius circles. Attached rendering by original master and after your patch. Thanks!

[tuupola]

Seems filled circle needs some adjusting too. Image generated with:

hagl_draw_circle(backend, DISPLAY_WIDTH / 2, DISPLAY_WIDTH / 2, 300, red);
hagl_fill_circle(backend, DISPLAY_WIDTH / 2, DISPLAY_WIDTH / 2, 300, blue);