Add touch controls

Add touch controls to my game, keeping keyboard working: <paste input
handling>

- <Virtual stick / swipe / tap zones> — recommend which suits my game
  and say why.
- Controls sized for thumbs and positioned where thumbs actually rest,
  not centred at the bottom.
- Handle multi-touch: moving and jumping at the same time must work.
- Prevent the browser's defaults — scrolling, pull-to-refresh,
  double-tap zoom, text selection — inside the game area only.
- Hide touch controls when a keyboard is used, and vice versa.

Also handle the screen rotating mid-game without breaking the canvas.