monitor-manager: Remove 'scale' from MetaOutput

Replace the 'scale' of an output with a vfunc on the MetaMonitorManager
class that takes a monitor and a monitor mode which calculates the
scale. On X11 this always returns 1, on KMS, the old formula is used.
On the dummy and test backends, the already configured values are
returned.

https://bugzilla.gnome.org/show_bug.cgi?id=777732

src/backends/meta-monitor-manager.c

@@ -275,6 +275,19 @@ meta_monitor_manager_is_headless (MetaMonitorManager *manager)
   return !manager->monitors;
 }
 
+int
+meta_monitor_manager_calculate_monitor_mode_scale (MetaMonitorManager *manager,
+                                                   MetaMonitor        *monitor,
+                                                   MetaMonitorMode    *monitor_mode)
+{
+  MetaMonitorManagerClass *manager_class =
+    META_MONITOR_MANAGER_GET_CLASS (manager);
+
+  return manager_class->calculate_monitor_mode_scale (manager,
+                                                      monitor,
+                                                      monitor_mode);
+}
+
 static void
 meta_monitor_manager_ensure_initial_config (MetaMonitorManager *manager)
 {