- For webkit engines, do array manipulation for each tile subrectangle
  and only use the array for putImageData after rendering is finished.

In Chrome 5.0.375.29 beta, the time to render a full 800x600 hextile
image dropped from 500ms to 250ms or so. Firefox 3.5.3 rendering of
a full 800x600 hextile image is about 2300ms.