Fix broken ceilings

[bgaesslein]

• Refactored methods for creating floors and ceiling to reduce duplication
• Introduced a method that flips ceilings

Using reverse() to flip the vertices and uv lists leads to holes in the ceiling. To remedy this, I instead use the Vector3 Scale function to mirror the whole plane and then adjust the y-value because it is being mirrored along the horizontal-zero-plane.

[bgaesslein]

There are still some ceilings with small gaps but in those rooms the floor also has a gap in the same place. So some further debugging is in order. Most ceilings are fixed tho.