Add canvas.drawCircle()

[vtereshkov]

No description provided.

[marekmaskarinec]

@skejeton we should think about this when designing the new renderer.

[skejeton]

I was thinking whether Tophat could run well on software renderer - for now, the main bottleneck in the performance is not actually rendering stuff to the screen, but incorrectly using the GPU pipeline, software rendering doesn't have this issue because you can directly write to the memory without any pipelines. It'll also allow to implement shapes like circles without the need for triangles and LOD/generating every time.