Push get_buffer_requests_recursively into rust-layers

Also do some reorganization and abstraction in IOCompositor now that we
do not have to borrow self.scene.root.
Rename Compositor::ask_for_tiles to send_buffer_requests_for_all_layers

This name better reflects what the method does.
Remove CompositorData::add_child

The contents of this method have dwindled to two statements. Just
remove the method now.
Merge pull request #2976 from mrobinson/more-cleanup

Move get_buffer_requests into rust-layers from Servo and do some minor cleanup