textview: Don't leave embedded children behind

When scrolling embedded widgets out of view,
they sometimes get left behind because we don't
reallocated them. To avoid that, move _all_ children
out of view in size_allocate, and let the current
child allocation plumbing move the visible ones
back in place.

demos/gtk-demo/hypertext.c

@@ -94,13 +94,12 @@ show_page (GtkTextView *text_view,
       gtk_text_buffer_insert (buffer, &iter, " can easily be realized with ", -1);
       insert_link (buffer, &iter, "tags", 2);
       gtk_text_buffer_insert (buffer, &iter, ".\n", -1);
-      gtk_text_buffer_insert (buffer, &iter,
-          "Of course you can also embed Emoji 😋, "
-          "icons ", -1);
+      gtk_text_buffer_insert (buffer, &iter, "Of course you can also embed Emoji 😋, ", -1);
+      gtk_text_buffer_insert (buffer, &iter, "icons ", -1);
 
       theme = gtk_icon_theme_get_for_display (gtk_widget_get_display (GTK_WIDGET (text_view)));
       icon = gtk_icon_theme_lookup_icon (theme,
-                                         "microphone-sensitivity-high-symbolic",
+                                         "eye-not-looking-symbolic",
                                          NULL,
                                          16,
                                          1,

gtk/gtktextview.c

@@ -4409,6 +4409,8 @@ gtk_text_view_allocate_children (GtkTextView *text_view)
     {
       const AnchoredChild *child = iter->data;
       GtkTextIter child_loc;
+      GtkRequisition child_req;
+      GtkAllocation allocation;
 
       /* We need to force-validate the regions containing children. */
       gtk_text_buffer_get_iter_at_child_anchor (get_buffer (text_view),
@@ -4429,6 +4431,15 @@ gtk_text_view_allocate_children (GtkTextView *text_view)
         }
 
       gtk_text_layout_validate_yrange (priv->layout, &child_loc, 0, 1);
+
+      gtk_widget_get_preferred_size (child->widget, &child_req, NULL);
+
+      allocation.x = - child_req.width;
+      allocation.y = - child_req.height;
+      allocation.width = child_req.width;
+      allocation.height = child_req.height;
+
+      gtk_widget_size_allocate (child->widget, &allocation, -1);
     }
 }
 
@@ -5910,9 +5921,6 @@ gtk_text_view_snapshot (GtkWidget   *widget,
   snapshot_text_view_child (widget, priv->bottom_child, snapshot);
   snapshot_text_view_child (widget, priv->center_child, snapshot);
 
-  /* Propagate exposes to all unanchored children. 
-   * Anchored children are handled in gtk_text_view_paint(). 
-   */
   for (iter = priv->anchored_children.head; iter; iter = iter->next)
     {
       const AnchoredChild *vc = iter->data;