Blender Git Loki

Cycles: Introduce scene host_update function

The longer-term goal is to separate host-only scene update
from device update: make it possible to make kernel features
depend on actual scene state and flags.

This change makes it so shaders are compiled before kernel
load, making checks like "has_volume" available at the
kernel features calculation state.

No functional changes are expected at this point.

Differential Revision: https://developer.blender.org/D12795