Make mapmaker able to choose more materials or textures

[Poikilos]

Make a way to make multiple tiles in same dungeon. For transitions, maybe allow layers (overlay the corner & edge textures with transparent parts and make temp textures in memory with the result). In otherwords, instead of just choosing a texture for ground, make multiple materials allowed in the same world.

Textures with normal maps:

• https://opengameart.org/content/bricks-and-tiles (CC0)
• https://opengameart.org/content/tileable-bricks-ground-textures-set-1 (CC0)
• https://opengameart.org/content/tileable-brick-ground-textures-set-2 (CC0)
• https://opengameart.org/content/old-brick-wall (CC BY-SA 3.0)
• https://opengameart.org/content/stone001-texture-seamless (CC BY 4.0)
• https://opengameart.org/content/basalt-tiles (CC0)
• https://opengameart.org/content/sandstone-tiles (CC0)

Textures (need normal map generation, such as using AwesomeBump):

• https://opengameart.org/content/rough-coloured-concrete (CC BY 3.0)
• https://opengameart.org/content/new-england-textures-i (CC0, has bump but not normal)

[Upabjojr]

Assessment: the editor's tile palette is driven by the same tileset config as the game (config/tilesets.cfg), so letting the mapmaker choose more materials mostly means exposing the tileset's variants in the editor UI — a real feature, needing editor UI work rather than engine changes. The recent editor-improvements PRs (#22) are the natural place to build this on.