undo an ancient hack; be an idiomatic LÖVE app

Dependencies

• [2] AGJXIDOF hardcode some assumptions about how this app uses love
• [3] DJSIRUMD a more radical attempt at ignoring nil y's
• [4] 3QNOKBFM beginnings of a test harness
• [5] AVTNUQYR basic test-enabled framework

Change contents

• replacement in app.lua at line 35
  [3.551]→[2.124:186](∅→∅)
  − -- draw before update to give it a chance to mutate state
  [3.551]

  [2.186]
  + dt = love.timer.step()

  + App.update(dt)

  +
• edit in app.lua at line 43
  [3.637]→[2.317:363](∅→∅)
  − dt = love.timer.step()

  − App.update(dt)