Add object_distance function

We can now easily calculate the distance between objects.

Signed-off-by: Casey Dahlin <cdahlin@redhat.com>
Add pre_skip_children

When pre-iterating objects, we occasionally would like to skip the current
object as well as its children. This will advance the cursor such that that
will happen, and otherwise functions as a continue.

Signed-off-by: Casey Dahlin <cdahlin@redhat.com>
Add draw distance API

Signed-off-by: Casey Dahlin <cdahlin@redhat.com>