* Added a --window flag to the game executable to specify the startup
window resolution. Either specify an exact size (e.g. 1024x768) or a
special keyword "desktop", "mobile" or "landscape"
* Default size is "desktop" or 1024x768
* Mobile simulates a smartphone form factor of 375x812
* Landscape is mobile but flipped horizontally.
* Doodle UX isn't great on mobile but this is a step towards working on
mobile friendly UI
