Dark UI Mode, compact commands

• Viewer commands now use std::variant, reducing wasted space for unused command data
• Added the glow::viewer::dark_ui configure tag, switching the UI theme to a dark version
  • No nesting, any single ViewerApp only has one UI mode (unspecified ordering if used more than once within the same main command queue)
  • Also has a glow::viewer::dark_ui(false) version
• Fixed a crash on the Shift + Esc global termination due to a missing AsyncTextureLoader shutdown
• The camera now rests in its initial position instead of smoothing to it on startup