xdg-shell: Turn the resizing heuristics into an explicit state

Currently, there's a race condition. When resizing from the left, and
a client attaches a buffer after the resize ends, you suddenly see the
buffer jump to the right, because the resize ended while multiple
attaches were in-flight. Making resize a state can fix this, as the
server can now know exactly when the resize ended, and whether a commit
was before or after that place.

We don't implement the correct tracking in this commit; that's left as
an exercise to the reader.

Additionally, clients like terminals might want to display resize popups
to display the number of cells when in a resize. They can use the hint
here to figure out whether they are resizing.

protocol/xdg-shell.xml

@@ -270,6 +270,12 @@
         The surface is fullscreen. The window geometry specified in the configure
         event must be obeyed by the client.
       </entry>
+      <entry name="resizing" value="3">
+        The surface is being resized. The window geometry specified in the
+        configure event is a maximum; the client cannot resize beyond it.
+        Clients that have aspect ratio or cell sizing configuration can use
+        a smaller size, however.
+      </entry>
     </enum>
 
     <event name="configure">