Allow brush entity displacements

[vrad-exe]

What would this enhancement be for?

Tools

Describe your enhancement suggestion in more detail

Displacements should be allowed to be attached to brush entities, as currently VBSP will prevent this. I remember hearing this ability was originally present in the HL2 beta (alongside the infamous env_terrainmorph), but was removed for whatever reason.

This is separate from #1002 because it will likely require more substantial changes to allow displacements to move.

[MyGamepedia]

env_terrainmorph in Strata games would be incredibly cool, because the only what we can do with surface distortion is this. Here e3 demo.

[vrad-exe]

Ok but that's not what this issue is about, please stop replying to issues with off topic things