Add user-toggleable mesh face culling

[adamkewley]

The UI currently uses no face culling because various data sources (e.g. custom scripts generating meshes from CT data, etc. etc.) can have sketchy vertex winding,which causes gaps in the mesh.

However, face culling can be useful if for looking inside correct meshes.

The situation in which you'd probably want to view inside a bone, rather than outside, is when the species being observed has an exoskeleton (e.g. an insect)