widget: Add gtk_widget_set_transform

This might not stay until the end but for now it's good enough to test
widget transforms.

gtk/gtkwidget.c

@@ -13712,3 +13712,12 @@ gtk_widget_get_height (GtkWidget *widget)
          border.top  - border.bottom -
          padding.top - padding.bottom;
 }
+
+void
+gtk_widget_set_transform (GtkWidget               *widget,
+                          const graphene_matrix_t *transform)
+{
+  GtkWidgetPrivate *priv = gtk_widget_get_instance_private (widget);
+
+  graphene_matrix_init_from_matrix (&priv->transform, transform);
+}

gtk/gtkwidget.h

@@ -1050,6 +1050,10 @@ void                    gtk_widget_snapshot_child       (GtkWidget   *widget,
                                                          GtkWidget   *child,
                                                          GtkSnapshot *snapshot);
 
+GDK_AVAILABLE_IN_ALL
+void                    gtk_widget_set_transform        (GtkWidget               *widget,
+                                                         const graphene_matrix_t *transform);
+
 G_DEFINE_AUTOPTR_CLEANUP_FUNC(GtkWidget, g_object_unref)
 G_DEFINE_AUTOPTR_CLEANUP_FUNC(GtkRequisition, gtk_requisition_free)