Enhancement: Add planar reflections to PBR shader

[TheEnderek0]

What would this enhancement be for?

Rendering

Describe your enhancement suggestion in more detail

Planar reflections are already possible in LightmappedReflective
These basically re-render the world again, like in the Water shader. This would be extremely useful for areas that require these high quality reflections, like a window in a lit room. However, it would also require to increase the limit of allowed re-renders (cameras) active at a time, as currently only one (not including Portals) is allowed, hence if you try to use two LightmappedReflective materials only one of them is going to work properly.

Additionally, there could a way to disable specific brushes via a proxy of some sorts, like material_modify_control but for a specific brush only, or in general not re-rendering the world if the surface is not visible. This would decrease the performance impact if used carefully.