Implement lightmaps

[ThomasMountainborn]

The indoor spaces are all lit by lightmaps.

• Get the unique lightmap textures from ZenKit, as there are many LightMap struct instances referring to the same texture, but with different coordinates.
• Group the polygons using the same lightmap together.
• Demystify the Zen lightmap coordinates and apply the lightmap correctly to the geometry.
• Make lightmaps work in all scenes.
• Check if indoor scenes can just use the vertex colors instead.
• Evaluate the performance impact of not merging objects with different lightmaps.
• Evaluate the performance impact of not merging lightmapped and non-lightmapped objects.