Devlog 1.5: Optimisation and tweaks

What's new:

• Rotated the models to match Unity and Unreal Engine axis orientation.
• Minor texture and geometry topology tweaks and optimisations.
• Added some other variants for the modular structures.
• Added some new textures to use for the floor.
• Fixed flipped normals here and there.
• Made so the structures such as walls and floors would have different material channels for top and bottom, so the other side of a wall for example could have a different texture.