Gameplay fixes

Version 1.4 fixes four issues:

• Gameplay: Square tetrominos can not be rotated anymore, this annoyed some players before.
• Gameplay: Key-repeat behaves differently now, with an initial delay for the first repeat, and then faster repeats after that. This feature is completely custom - I first hoped that I could rely on the "auto key" event in Mac OS, but that doesn't fire for every held button, so if you want key-repeat for the left arrow key and press the rotate button once, the tetromino would stop moving which felt very jarring. I like it this way instead, where the left/right arrow keys have key repeat and the other buttons do not affect them.
• Rendering: There was a bug where the menu would sometimes render front-to-back instead of back-to-front, which made it impossible to read the text. It only happened one some computers, and only sometimes. I think I fixed it now.
• Performance: Preloading the sound effects to prevent stuttering.

Let me know if you have more suggestions for improvements - in the meantime I will work on further performance optimization. We ain't done yet!