Mac and Windows icons

Icons are now being generated and assigned to Mac and Windows builds!

On Windows i'm using makeicon, a handy little command line script created by an ex-colleague for generating .ico files. Then rcedit to assign the icon to the exe.

On mac i'm currently using a shell script similar to this stackoverflow answer, however I may also switch that to makeicon and bundle it in the repo just because it reduces an external dependency.

The scripts expect a single 1024x1024 png file called `icon.png` to exist in `assets/source` as their input.

Nothing crazy but having an icon for your game defo makes you feel more legit :)