General improvements, mostly for macOS

[zydeco]

I've implemented the suggestions in #10 and a few more improvements:

General improvements:

• Use GL_NEAREST for upscaling: will not be blurry on retina screens
• Preload more lightning sounds: while not playing, you can click faster before the sound fails to play
• Keep score and level on bottom bar after the game is over: I chose to hide the lives since the game is over when you have no lives left

macOS-specific:

• Allow resizing the window to integer multiples of the game size (to keep pixel-perfect drawing, including ½ on retina screens)
• Hide cursor while playing, even if it is moved (while playing, it re-hides on keyboard input)
• Full screen enhancements
  • Scales up to nearest integer multiple of the game size (to keep pixel-perfect drawing)
  • Hide the menubar