Use a single workspace in PlanarReflections?

[jwwalker]

For background, see: https://forums.ogre3d.org/viewtopic.php?t=97437&sid=add90650d9ca8b1d7870313b83d9534a

In summary, PlanarReflections::setMaxActiveActors creates a separate workspace instance for each "actor" (mirror). Since shadow nodes are not shared among workspaces, this can result in heavy use of texture memory. So it would be nice if someone could redesign things to use just one workspace.