Changes:

• After hitting a shadow catcher, re-initialize the volume stack taking into account shadow catcher ray visibility. This ensures that volume objects are included in the stack only if they are shadow catchers.
• If there is a volume to be shaded in front of the shadow catcher, the split is now performed in the shade_volume kernel after volume shading is done.
• Previously the background pass behind a shadow catcher was done as part of the regular path, now it is done as part of the shadow catcher path.

For a shadow catcher path with volumes and visible background, operations are
done in this order now:

• intersect_closest
• shade_volume
• shadow catcher split
• intersect_volume_stack
• shade_background
• shade_surface

The world volume is shaded in both the shadow and non-shadow catcher paths
now, which means the fog is assumed to already exist in the footage. We may
consider adding an option to control this, or change the default. With a
volume object this control is already possible.

This includes refactoring to centralize the logic for next kernel scheduling
in intersect_closest.h.