[r2] Single-player and cooperative modes, small fixes, web build

Changes:

• I have added a single-player mode!
  Or, rather, re-enabled it and allowed all levels to be selected in it.
  I also made the game remember your best score per level.
• Added a cooperative mode toggle.
  When enabled, the game waits until everyone is dead before restarting the level.
• You may now switch to fullscreen via Alt+Enter.
• Added < > arrows to level screen UI
  (previously it was unclear that there are multiple levels to choose from)
• Added input buffering for jump/hit actions.
  (this means that you can press the button a little early and not die)
• Reduced jump cooldown from 1/2s to 1/3s.
  (that's still more than enough time to get hit by a disc)
• Added a web version.
  (previously affected by a mystery rendering issue)