Mountains

**Please do not merge this yet.**

This pull request is to keep track of what still need to be done to be able to complete #636 and hopefully get some help from those more knowledgable on how to solve some issues.

- [ ] I am yet to test this code with the AI but if the AI does not build mines properly then I will need help on AI coding.
- [ ] The graphics for the mine are placed on top of the ground tiles. I can use transparency to fake them being behind mountains sometimes. but for some use cases this does not work:
 - Near corners
 - If we allow players to build single height mountains as well as double height then graphics for the mine will be very wrong

My Ideas for sorting this currently are:
- Fake the mine being a building completely and have it drawn in the ground layer (very hacky)
- Allow different graphics to be used by the mine in different situations (pretty hacky)
- Enable some kind of z-ordering between layers (I'm guessing this would hit performance?)
- Fake 'mountain buildings' and match these to the tiles beneath so that the z-ordering works on the building layer, and mines are then drawn behind the mountain buildings

I will have a similar problem to the mines when showing deposit graphics, though probably less so.