Step two: draw some trees and bushes

This code draws a border of 'trees' and a random spattering of 'bushes'
on the canvas.

The entity-component piece appears to revolve around instantiating
entities, which themselves have components... so "canvas" is a component
indicating it can be drawn on a canvas.

Seems like setting the attributes is the pattern I was vaguely familiar
with from I believe a lighttable talk. Something there about just
pumping updates to attributes to the entities, rather than full scale
painting. Dunno but sounds like something I could get my head around.

Things I don't like:

- The grid thing... suprised this is not built in but maybe too
  specialised/trivial. I would make this a separate object but
understand keeping the tutorial simple

- The double for loop (see earlier notes re the simplicity for tutorial)

- Doing the grid calculations manually: I like how d3 does this. I
  wonder if d3 is a good fit not for the graphics but for the
translation/projection stuff. Early days as crafty could come with this
stuff.