Don’t induce a stroke if we won’t see it

By specifying the line color, we were causing nodetiles to send stroke commands to canvas. Even with a line width of 0, it seems there was a performance penalty.

With test URLs from [here](https://gist.github.com/prashtx/6da975d94803b06a3249/raw/df629a2f9ea5b19b073a938b8debed24f5316a6b/tiles-userflow-206.json), we save about 4.9% on the mean response time, maybe 3.5% on perc99, and apparently 17.5% on the max response time.

http://bit.ly/1pz6kHE