Add a synthetic-lines example

This PR adds a synthetic-lines example whose goal is to evaluate the performance of drawing lines. The example page includes a table with fps numbers for the devices/browsers I have handy. Please add yours.

Executive summary:

* Chrome is very fast. I get a solid 60 fps while panning with 20000 lines.
* Safari freezes at 10000 lines.
* FireFox is in between.
* Mobile devices/browsers are much slower.