nitpicker: prevent the pointer from going nowhere

The 'size_changed' handling remained without effect when the (last)
display re-appears (e.g., back from screen blank) because
'Capture_root::bounding_box()' delivered the '_fallback_bounding_box'
in the intermediate phase where no display was present. Unfortunately,
'Capture_root::visible()' failed to apply the same logic. This patch
makes 'visible()' consistent with 'bounding_box()'. It has the
welcome effect that nitpicker remembers the pointer position during
the dark phase.

Fixes #5397

repos/os/src/server/nitpicker/main.cc

@@ -319,7 +319,10 @@ class Nitpicker::Capture_root : public Root_component<Capture_session>
 				[&] (Point const p) {
 					_sessions.for_each([&] (Capture_session const &session) {
 						if (!result && session.bounding_box().contains(p))
-							result = true; }); },
+							result = true; });
+					if (!result)
+						result = _fallback_bounding_box.contains(p);
+				},
 				[&] (Nowhere) { });
 			return result;
 		}