add debug overlay grid, togglable with F4

Inspired on  #135.

This simply draws 2d lines over the screen: it has no fancy shaders or anything. It won't work once we have terrain altitude. But it has helped me to spot what I think it's a bug on the coordinate conversion code :D also, it's useful for fixing #57.
I _think_ somewhere in our style guide it says to always put code blocks in the next line, even if it's just one statement, i.e.

  if (condition) {
    statement()
  }