Fix issue with FMeshNodes and visibility

[MattRix]

from an email: "make two mesh nodes, add them next to each other, add some triangles, do some stuff. Later, make the second one invisible (without changing its data and calling MarkChanged) - it ignores the isVisible change to false. I haven't fully traced it through, but it might be that the mesh node needs the call to MarkChanged to make it repopulate the render layer and respect the isVisible/visibleScale change in FNode. Without tracing it all, some dirty flags somewhere are getting stomped I suppose, or the order of all the population stuff doesn't quite work for mesh nodes. I can force a MarkChanged on the second meshnode when I want to set it invisible and then it works fine."