cursor-tracker: Add way to force tracking cursor position

On X11 we won't always receive cursor positions, as some other client
might have grabbed the pointer (e.g. for implementing a popup menu). To
make screen casting show a somewhat correct cursor position, we need to
actively poll the X server about the current cursor position.

We only really want to do this when screen casting or taking a
screenshot, so add an API that forces the cursor tracker to track the
cursor position.

On the native backend this is a no-op as we by default always track the
cursor position anyway.

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1391

clutter/clutter/clutter-private.h

@@ -313,6 +313,12 @@ us (uint64_t us)
   return us;
 }
 
+static inline uint32_t
+ms (uint32_t ms)
+{
+  return ms;
+}
+
 static inline uint64_t
 ms2us (uint64_t ms)
 {