Attempt to fix Linux and MacOS builds

Attempted to fix the Mac and Linux builds.

- Replaced System.DateTime, with simply a float. Timings of hit stun and enemy AI could be different from Windows builds. Played briefly and it feels similar.

- Added fullscreen toggle and resolution selector in the top left corner to hopefully fix the screen resolution issue seen on a linux box. Not sure what the issue is so I'm just hoping toggling will help.

- Added two smaller resolutions in case the person testing the game happens to be playing on an ancient monitor.