1. use tree slim's merge strategy in hybrid support
2. do not break bridge in hybrid support
3. fix "support on buildplate only" not working. The cause is: with
independent support layer height, contact_nodes are not synced with
object layers, so the previous way of accessing contact nodes in
unsupported_branch_leaves no longer works. To fix that, we add
obj_layer_nr.
Jira: STUDIO-2250
Change-Id: Idfc9b0bb10425de6414f7eae2cf5467cedf1d669
(cherry picked from commit 8780d5a7e92e298cbbccbb3bd1825197d387a340)
305121a233