Fix MeshInstance3D gizmo redraw performance for PlaneMesh with larger subdiv value

[smix8]

The MeshInstance3D editor gizmo had really low performance when the node had a PlaneMesh with larger subdiv value.

This caused noticeable editor frame stutters everytime the node was selected or unselected, the mesh was changed, or just an input happened close to the plane AABB.

E.g. a 100+ subdiv PlaneMesh for terrain / water would cause noticeable performance issues with each update. A 1000+ subdiv would freeze the entire editor for multiple seconds. With this change the update is almost instant again.

It was so slow due to every call to gizmo redraw creating a new TriangleMesh for the PlaneMesh gizmo collision.

The TriangleMesh did exessive BVH tree building and AABB calculations for every single mesh face and an average subdiv plane mesh had easily a few thousands of them. For a specialized Mesh type that can only be a flat plane no matter the subdiv this is totally unnecessary and 2 faces is enough for the input collision that the editor needs.

[clayjohn]

Makes sense to me. I wish there was an elegant way to expand this for all PrimitiveMeshes. But for now, it makes sense to do for PlaneMesh as that is the most common one to have a huge subdivision amount.

[akien-mga]

Thanks!