display: Add meta_display_get_monitor_scale

This will return the monitor scaling for the requested logical screen

https://bugzilla.gnome.org/show_bug.cgi?id=765011

src/core/display.c

@@ -3513,6 +3513,37 @@ meta_display_get_monitor_geometry (MetaDisplay   *display,
   *geometry = logical_monitor->rect;
 }
 
+/**
+ * meta_display_get_monitor_scale:
+ * @display: a #MetaDisplay
+ * @monitor: the monitor number
+ *
+ * Gets the monitor scaling value for the given @monitor.
+ *
+ * Return value: the monitor scaling value
+ */
+float
+meta_display_get_monitor_scale (MetaDisplay *display,
+                                int          monitor)
+{
+  MetaBackend *backend = meta_get_backend ();
+  MetaMonitorManager *monitor_manager =
+    meta_backend_get_monitor_manager (backend);
+  MetaLogicalMonitor *logical_monitor;
+#ifndef G_DISABLE_CHECKS
+  int n_logical_monitors =
+    meta_monitor_manager_get_num_logical_monitors (monitor_manager);
+#endif
+
+  g_return_val_if_fail (META_IS_DISPLAY (display), 1.0f);
+  g_return_val_if_fail (monitor >= 0 && monitor < n_logical_monitors, 1.0f);
+
+  logical_monitor =
+    meta_monitor_manager_get_logical_monitor_from_number (monitor_manager,
+                                                          monitor);
+  return logical_monitor->scale;
+}
+
 /**
  * meta_display_get_monitor_in_fullscreen:
  * @display: a #MetaDisplay

src/meta/display.h

@@ -246,6 +246,10 @@ void meta_display_get_monitor_geometry (MetaDisplay   *display,
                                         int            monitor,
                                         MetaRectangle *geometry);
 
+META_EXPORT
+float meta_display_get_monitor_scale (MetaDisplay *display,
+                                      int          monitor);
+
 META_EXPORT
 gboolean meta_display_get_monitor_in_fullscreen (MetaDisplay *display,
                                                  int          monitor);