dragicon: Handle compute-size

Compute our size when requested by the backend. This makes GtkDragIcons
actually recompute their size when it changes, instead of getting stuck
with the first size and potentially underallocating.

gtk/gtkdragicon.c

@@ -28,6 +28,7 @@
 #include "gtkcssnumbervalueprivate.h"
 
 #include "gdk/gdksurfaceprivate.h"
+#include "gdk/gdkdragsurfacesize.h"
 
 /* for the drag icons */
 #include "gtkcolorswatchprivate.h"
@@ -183,6 +184,16 @@ surface_render (GdkSurface     *surface,
   return TRUE;
 }
 
+static void
+surface_compute_size (GdkDragSurface     *surface,
+                      GdkDragSurfaceSize *size,
+                      GtkWidget          *widget)
+{
+  GtkRequisition nat_size;
+  gtk_widget_get_preferred_size (widget, NULL, &nat_size);
+  gdk_drag_surface_size_set_size (size, nat_size.width, nat_size.height);
+}
+
 static void
 gtk_drag_icon_realize (GtkWidget *widget)
 {
@@ -193,6 +204,7 @@ gtk_drag_icon_realize (GtkWidget *widget)
   gdk_surface_set_widget (icon->surface, widget);
 
   g_signal_connect (icon->surface, "render", G_CALLBACK (surface_render), widget);
+  g_signal_connect (icon->surface, "compute-size", G_CALLBACK (surface_compute_size), widget);
 
   GTK_WIDGET_CLASS (gtk_drag_icon_parent_class)->realize (widget);
 
@@ -216,6 +228,7 @@ gtk_drag_icon_unrealize (GtkWidget *widget)
   if (icon->surface)
     {
       g_signal_handlers_disconnect_by_func (icon->surface, surface_render, widget);
+      g_signal_handlers_disconnect_by_func (icon->surface, surface_compute_size, widget);
       gdk_surface_set_widget (icon->surface, NULL);
     }
 }