feat: splashscreen, multiple windows

[craftablescience]

https://user-images.githubusercontent.com/26600014/230776360-fce0c31d-1062-4ae5-8802-22b3c5409241.mp4

• Adds a 640x480 splashscreen window before the main window is created at the end of initialization
  • Since this allows us to get an OpenGL context, it is not optional, but it can optionally be hidden
  • This should hopefully fix the splashscreen not showing on OSX, but probably needs more work
• Rewrite Device api to resemble Backend api and handle ImGui + input there instead of Engine::run
• Allow for multiple windows to be created
• Fix an inverted not operator in UBO handle