v0.7.1 — zoom no longer loses the city

Two things a stranger hit within minutes of arriving.

Fixed

• The screen no longer goes blank when you zoom in. Reported from Hacker
  News: "the site is going blank sometimes if I zoom in a bit too much."
  Reproduced by sweeping the camera through its dolly range and reading every
  frame — from 16 units inward the readable city disappears, and at 12 and 8
  units the canvas is a near-flat ground or roof surface. The camera was being
  allowed to dolly inside the buildings it was looking at, so every surface in
  frame was back-faced and there was nothing to see, with no indication of what
  had happened or how to get back out.
• The rotate gesture is discoverable. Someone asked "How to rotate camera?"
  — the controls follow the Google Maps convention, where left-drag pans and
  shift with left-drag rotates, which is a good scheme that nobody could find.
  That was the sixth time in this project that something has been built, wired
  and left invisible, so the hint now appears where the gesture is being
  attempted rather than in the help overlay where the previous five already
  were.