* compositor.h: Update prototypes.
* idle_inhibit.c (DetectSurfaceIdleInhibit): Handle subsurfaces;
when an inhibitor attached to a subsurface is displayed, check
if the subsurface's root is focused or not.
(CreateInhibitor): Likewise; handle subsurfaces specially.
* subcompositor.c (ViewisVisible): New function.
* subsurface.c (struct _Subsurface): New field `mapped'.
(Commit, Teardown): Update idle inhibition when the surface is
mapped or unmapped.
(XLSubsurfaceGetRoot): New function.