Allow loading a Tileset without a Map

[mpartel]

Useful e.g. for generating maps in code.

Includes some cleanup commits.

Not super extensively tested, since I don't have a large project using this yet.

[fallahn]

Hi! Thanks for this. I'm a little reluctant to add this right now if you say it's not fully tested - however please do keep this up to date with any changes so I can re-evaluate it down the line! 😁 If the clean up commits are important (such as the Colour operator) you can always move them to a separate PR.

[mpartel]

I'm actually not sure where the bar should be for testing :thinking: The change works for me, and it doesn't break the included examples :man_shrugging: If it also works with your games, and passes your review, perhaps that's good enough?

Anyway, no pressure, I don't mind using a forked version in my project.

[fallahn]

Sure! I'll give it a go when I have a moment - if nothing is broken then I'm sure it'll be OK to merge 😁