Don't call rut_simple_refable_(un)?ref directly

Calling the ref implementation functions directly instead of via
rut_refable_(un)?ref messes up the reference count debugging and it
also seems a bit less robust.
rut-entry: Don't ref the context on free

_rut_entry_free was refing the RutContext. Presumably this is a typo
and it is meant to unref.
Fix some leaking references

This cleans up some resources allocated by main.c and RigCameraView.
This is enough to make it usually remove all of the references on the
CoglOnscreen although it looks something can cause it to still leak
such as adding a new component.