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

@@ -25,7 +25,7 @@
     THIS SOFTWARE.
   </copyright>
 
-  <interface name="zwlr_layer_shell_v1" version="2">
+  <interface name="zwlr_layer_shell_v1" version="3">
     <description summary="create surfaces that are layers of the desktop">
       Clients can use this interface to assign the surface_layer role to
       wl_surfaces. Such surfaces are assigned to a "layer" of the output and
@@ -82,9 +82,37 @@
       <entry name="top" value="2"/>
       <entry name="overlay" value="3"/>
     </enum>
+
+    <!-- Version 3 additions -->
+
+    <event name="new_output" since="3">
+      <description summary="notify of a new wl_output">
+        This event indicates that a new output has been created. If the 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.
+
+        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="2">
+  <interface name="zwlr_layer_surface_v1" version="3">
     <description summary="layer metadata interface">
       An interface that may be implemented by a wl_surface, for surfaces that
       are designed to be rendered as a layer of a stacked desktop-like