st/vega: fix vg_context_is_object_valid()

st/vega: fix vg_context_is_object_valid()

vg_context_is_object_valid() checks if a handle is valid by checking if
the handle is a valid key of the object hash table.  However, the keys
of the object hash table were object pointers.

Fix vg_context_add_object() to use the handles as the keys so that
vg_context_is_object_valid() works.  This bug was introduced by
99c67f27d35a4bbbbefada8117d5972c7583cf42.