Add a debug interface to translate from surface to screen coordinates.
This is the Mir part of the infrastructure for Autopilot to determine the screen location of widgets, for full-stack testing.
This is hidden behind a --debug server option, to make it absolutely clear that applications cannot depend on this functionality outside of a constrained environment.
(See also https://lists.ubuntu.com/archives/mir-devel/2014-August/000848.html)