External PR by Karim Jerbi (@KarimJerbi).
- NAVVCUBE is a command toggle so it never set active_tool; add a
show_viewcube flag to the ribbon and special-case is_active_tool so the
button highlights only while the cube is on.
- show_viewcube was absent from render_signature, so toggling a cached
view produced an identical signature and left stale cube pixels; hash
it into the signature so the toggle invalidates the cache and clears.