Add height map support.
Improve voxel shadow rendering.
Account for terrain height in Viewport.VisibleCells.
WIP: Add terrain geometry visualisation.
WIP: Hook up basic city buildings.

TODO Footprints.
Introduce ActorPreviewWidget and associated plumbing.
WIP In-game map editor.
WIP: Use per-tile color in minimap.

TODO: Properly fix missing tile definitions (which currently give invisible pixels).
Add demo map used for news screenshots.
WIP: Super hacky map importer, using format code from ccmaps-net.