Two-phase occlusion culling can be helpful for shadow maps just as it
can for a prepass, in order to reduce vertex and alpha mask fragment
shading overhead. This patch implements occlusion culling for shadow
maps from directional lights, when the `OcclusionCulling` component is
present on the entities containing the lights. Shadow maps from point
lights are deferred to a follow-up patch. Much of this patch involves
expanding the hierarchical Z-buffer to cover shadow maps in addition to
standard view depth buffers.
The `scene_viewer` example has been updated to add `OcclusionCulling` to
the directional light that it creates.
This improved the performance of the rend3 sci-fi test scene when
enabling shadows.
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