Expore improvements to minimap rendering performance

[Tyriar]

Currently we use putImageData to redraw the entire minimap each frame, we should explore keeping the source of truth of the image in the canvas and printing diffs to minimize the amount of rendering.

[Tyriar]

Not sure if there will be time for this in September (it's on the iteration plan though).

[alexdima]

I have pushed an optimization where the dirtyY and dirtyHeight arguments of putImageData are exercised to reduce in many cases how much painting is done.

@Tyriar I leave it up to you to explore further optimizations.