Improve game resolution settings

[miskatonicstudio]

Currently, GOAT only supports a fullscreen setting. It would be great to add different resolutions too. However, Godot doesn't support changing resolution in fullscreen and it seems that it is not going to be implemented soon (godotengine/godot#14542). However, running a game in fullscreen mode with the resolution of the screen (e.g. FullHD) causes the game to slow down significantly on older machines (because a lot more pixels have to be calculated). This can be solved by e.g. setting Godot resizing options (Project Settings) so for larger screen the game can still run in lower resolution, but the image will be scaled to fit the resolution of the screen.