The term "hdr" is so overloaded, we shouldn't use them anywhere, except
from maybe describing all of this work in blog posts and other marketing
materials.
So do renames:
* hdr => high_depth
* request_hdr => prefers_high_depth
This more accurately describes what is going on.
When sending render nodes from the client to the daemon we add an id,
and whenever we're about to re-send the entire tree node we instead
send the old id. We track all the nodes for the previous frame
of the surface this way.
Having the id on the daemon side will allow us do to much better deltas.
