Code review comment for lp:~mir-team/mir/vsync-timings-from-server

int64_t mir_surface_get_last_display_time(MirSurface *surface)
Somewhat related to my previous comment on this function, but is this exposing too much to the user? Is there some client-side processing that could be done in libmirclient without exposing the timing details?

The driving test looks like:
 2184 +TEST_F(VsyncProviderTest, last_display_time)
but could the test make use of mir client internals to check the vsync timing? (or, an uglier compromise could be for this to be in the debug library)