feat: support get scale by mmsg

protocols/dwl-ipc-unstable-v2.xml

@@ -241,6 +241,13 @@ I would probably just submit raphi's patchset but I don't think that would be po
       <arg name="keymode" type="string" summary="current keybind mode."/>
     </event>
 
+    <event name="scalefactor" since="2">
+      <description summary="scale factor of monitor.">
+        scale factor of monitor.
+      </description>
+      <arg name="scalefactor" type="uint" summary="scale factor of monitor."/>
+    </event>
+
   </interface>
 
 </protocol>