Don't resize guest display on zoom change

When the zoom level is changed, the virt-viewer window gets resized. But we
don't want this to trigger a resize of the guest display. But occasionally
rounding errors cause the guest display to be reconfigured when zooming out.  To
fix this, we first check whether the current size is the preferred size.  If it
is, we don't send down a resize command to the guest.

In addition to preventing guest resizes in response to zooming, it also improves
the behavior when the guest display resolution is changed from within the guest.
Before this change, we'd have the following behavior:
    A. guest changes display to WxH
    B. client gets notified of change and resizes the window to WxH
    C. client responds to window resize by sending a new monitor config command to the guest

With this change, the extra step C will be avoided because we're already at the
preferred size.

Resolves: rhbz#1004051

src/virt-viewer-display-spice.c

@@ -190,9 +190,22 @@ virt_viewer_display_spice_mouse_grab(SpiceDisplay *display G_GNUC_UNUSED,
 
 static void
 virt_viewer_display_spice_size_allocate(VirtViewerDisplaySpice *self,
-                                        GtkAllocation *allocation G_GNUC_UNUSED,
+                                        GtkAllocation *allocation,
                                         gpointer data G_GNUC_UNUSED)
 {
+    GtkRequisition preferred;
+    gtk_widget_get_preferred_size(GTK_WIDGET(self), NULL, &preferred);
+
+    /* when the window gets resized due to a change in zoom level, we don't want
+     * to re-size the guest display.  So if we get an allocation event that
+     * resizes the window to the size it already wants to be (based on desktop
+     * size and zoom level), just return early
+     */
+    if (preferred.width == allocation->width
+        && preferred.height == allocation->height) {
+        return;
+    }
+
     if (self->priv->auto_resize != AUTO_RESIZE_NEVER)
         virt_viewer_display_spice_monitor_geometry_changed(self);