layer-shell: introduce ack for new outputs

This ack sequence eliminates the race between a client creating a
new layer surface and the compositor rendering the first frame of a
new output.

unstable/wlr-layer-shell-unstable-v1.xml

@@ -92,6 +92,35 @@
         are not affected.
       </description>
     </request>
+
+    <event name="new_output" since="3">
+      <description summary="notify of a new wl_output">
+        This event indicates that a new output has been created. All clients
+        must send an ack_new_output request in response to this event. If a
+        client wishes to create a new layer surface in response to this new
+        output, it must make an ack_new_output request with the provided serial
+        after creating any new layer surfaces using the get_layer_surface
+        request and before the first commit on any of the new surfaces. If
+        the client does not wish to create a new layer surface, they should
+        ack immediately.
+
+        This allows the compositor to wait until new layer surfaces are ready
+        before rendering the first frame of the output.
+      </description>
+      <arg name="serial" type="uint"/>
+    </event>
+
+    <request name="ack_new_output" since="3">
+      <description summary="ack a new_output event">
+        This request informs the server that following commits on layer surfaces
+        created by the client take the new output into account.
+
+        A client may make multiple ack_new_output requests before committing.
+        The last request made before the commit indicates which new_output
+        event the commit is in response to.
+      </description>
+      <arg name="serial" type="uint" summary="serial from a new_output event"/>
+    </request>
   </interface>
 
   <interface name="zwlr_layer_surface_v1" version="3">