Add deactivate step to GLM slab processing.

Fixes numerous races involved in how terrabuff tried to clean up after
rendering.
Re-add drawing_method support to turtle.
Initial untested partial implementation of GL-based brush.

Not sure yet how to draw splotches efficiently and nicely.
Port tree prop to glbrush.

Brush still needs a lot of work. Also need to figure out why queueing the
geometry takes so long.