Add basic hiDPI support

[Calinou]

This upscales the game by a factor of 2 on hiDPI displays. The end result will be pixelated, but this makes the game playable on such displays.

[skyace65]

Isn't the proper way to do hiDPI with stretch settings? Though I have had issues getting those to work properly.

Also I wanted to thank you for all the pull requests. It's nice having someone new help with this.

[Calinou]

@skyace65 The game uses the disabled stretch mode right now. We could switch to the 2d stretch mode and increase the base resolution, but this may make things too large on larger displays. I have a Godot pull request that adds a custom scale factor to the 2d stretch mode, but it'll only be mergeable for 4.0.

Still, if we decide to use the 2d stretch mode, we can always multiply the camera zoom value by the automatic stretch factor to compensate.