Make turtles not pixelated and gross

Caching the turtle shapes made them pixelated and gross.

When you use `drawImage` to draw a scaled (smaller) version of one canvas into another, the quality degrades more than what you would expect for the target canvas's resolution (as in, more than if you just rendered the shapes directly to the target canvas). I don't know what this is; maybe the browser can't optimize the drawing in the same way since it's just trying to copy pixels. The solution to this is to simply increase the target canvas's resolution.

This pull request increases the resolution of the turtle canvas 3x. This value seemed to work the best for me, but this definitely should be tested on other monitors and other video cards. At 20x, my display freaked out and I had to hard restart, so I want to be sure this won't happen for those with a lesser or no video card.

I also increased the width of lines in turtles. This way, lines actually look like lines instead of a bunch of random pixels kind of in a row. You can now actually see the rays in the climate change model! This becomes a slight problem with turtles at large sizes: in NetLogo, lines remain thin, but here, the lines scale with the turtle. This addresses the observation at the end of issue #18.