PR #17684 broke occlusion culling because it neglected to set the
indirect parameter offsets for the late mesh preprocessing stage if the
work item buffers were already set. This PR moves the update of those
values to a new function, `init_work_item_buffers`, which is
unconditionally called for every phase every frame.
Note that there's some complexity in order to handle the case in which
occlusion culling was enabled on one frame and disabled on the next, or
vice versa. This was necessary in order to make the occlusion culling
toggle in the `occlusion_culling` example work again.
556b750782