Wayland: Update serial on button callback

The serial needs to be update for starting operations like moving and
resizing a window.

Closes #992.

src/wl_init.c

@@ -109,6 +109,8 @@ static void pointerHandleButton(void* data,
     if (!window)
         return;
 
+    _glfw.wl.pointerSerial = serial;
+
     /* Makes left, right and middle 0, 1 and 2. Overall order follows evdev
      * codes. */
     glfwButton = button - BTN_LEFT;