Prepare blend maps in the background.
Make sure lighting is correctly set from environ.
Allow optional populating for normals.
Make shadow more light weight and always present.

Data is only allocated when needed, since the shadow only is needed for
the fixed function pipeline.
Use pregen shadow texture for the Simple technique.

The fixed function pipeline requires a pregenerated shadow, supplied by
the TerrainPageShadow instance.
Update pregen shadows as time changes.

We currently recompile it each hour in game.