MVT: Add support for building extrusions

[gkjohnson]

Some options:

• Make a dedicated plugin based on the MVT buildings
• Adjust the MVTHierarchy traversal to ensure limited / no overlap between high LoDs and low LoDs to avoid visual conflict - improved settling perf could make this acceptable now. Then add support for buildings to the existing annotation system.
• Start with just basic meshes for buildings (then batched mesh later?)
• Trim the building shapes to the tile bounds