phosh-private: Add set_exclusive_zone_anchor request
This allows to specify which edge should the exclusive zone apply to for surfaces that are anchored to opposite edges.
This is a PoC implementation of an idea meant to tackle the issue described at https://github.com/swaywm/wlr-protocols/issues/102
Also contains a fix for a bug in exclusive zone calculation noticed while working on it.
In the future I'm going to propose a MR adding this feature to layer-shell. Meanwhile, this will allow phosh to run on unpatched wlroots (phosh#422).
An alternative approach would be to simply hardcode the anchor and apply it to surface with phosh home
namespace, but I wanted to try this approach first since I'd like it to eventually land in layer-shell itself.