Control Rig editor Preview viewport doesn't display static mesh attachments

Many of the animation asset editors don't display static mesh attachments in the way that the anim sequence or anim bp editor do.  This prevents users from previewing characters that are built from static meshes (eg. the mech in the valley of the ancient demo).

1. Open a skeletal mesh
2. Add a static mesh attachment to one of the bones
3. Create a control rig that uses this mesh as the preview mesh
   1. Note how the attachment is not displayed