voxel: Implement a closer-to-final vertex format

The vertex does not require us to run a GS any more (at the cost of extra
data for every vertex and losing ability to merge vertices in VB...).

Removing the GS means this can work on D3D9/GLES and is actually
noticeably faster even on hardware that supports GS.
voxel: Rework material table parsing

Now we have material information stored on CPU and layer information
stored on GPU.

This simplifies the shaders a bit and (more importantly) allows us to have
separate tiling parameters for side and top layers.

Also optimize current normal generation to get a better perf baseline.
Re-enable frame updates in unfocused state

We now throttle to 10 FPS to allow other apps to run interactively, but
still update simulation/rendering state to handle content reloads better.