prefs: Scale the root window cursor by the scale factor

This fixes small cursors on HiDPI displays.

src/core/prefs.c

@@ -1334,10 +1334,26 @@ meta_prefs_get_cursor_theme (void)
   return cursor_theme;
 }
 
+static int
+get_scale_factor (void)
+{
+  GdkScreen *screen;
+  GValue value = G_VALUE_INIT;
+
+  g_value_init (&value, G_TYPE_INT);
+
+  /* XXX: Should this be in ui/ ? Or MetaMonitorManager? */
+  screen = gdk_screen_get_default ();
+  if (gdk_screen_get_setting (screen, "gdk-window-scaling-factor", &value))
+    return g_value_get_int (&value);
+  else
+    return 1;
+}
+
 int
 meta_prefs_get_cursor_size (void)
 {
-  return cursor_size;
+  return cursor_size * get_scale_factor ();
 }