visibility-determining systems run.

The `check_visibility` system currently follows this algorithm:

1. Store all meshes that were visible last frame in
   the `PreviousVisibleMeshes` set.

2. Determine which meshes are visible. For each such visible mesh,
   remove it from `PreviousVisibleMeshes`.

3. Mark all meshes that remain in `PreviousVisibleMeshes` as invisible.

This algorithm would be correct if the `check_visibility` were the only
system that marked meshes visible. However, it's not: the shadow-related
systems `check_dir_light_mesh_visibility` and
`check_point_light_mesh_visibility` can as well. This results in the
following sequence of events for meshes that are in a shadow map but
*not* visible from a camera:

A. `check_visibility` runs, finds that no camera contains these meshes,
   and marks them hidden, which sets the changed flag.

B. `check_dir_light_mesh_visibility` and/or
   `check_point_light_mesh_visibility` run, discover that these meshes
   are visible in the shadow map, and marks them as visible, again
   setting the `ViewVisibility` changed flag.

C. During the extraction phase, the mesh extraction system sees that
   `ViewVisibility` is changed and re-extracts the mesh.

This is inefficient and results in needless work during rendering.

This patch fixes the issue in two ways:

* The `check_dir_light_mesh_visibility` and
  `check_point_light_mesh_visibility` systems now remove meshes that
  they discover from `PreviousVisibleMeshes`.

* Step (3) above has been moved from `check_visibility` to a separate
  system, `mark_newly_hidden_entities_invisible`. This system runs after
  all visibility-determining systems, ensuring that
  `PreviousVisibleMeshes` contains only those meshes that truly became
  invisible on this frame.

This fix dramatically improves the performance of [the Caldera
benchmark], when combined with several other patches I've submitted.