gimp_group_layer_update_size(): remove bogus optimization

The optimization here which would change only the group layer's offset
if the extents of the union of its children has not changed is
completely bogus. That case can only happen if one of the chldren was
moved in a way that does not change the extents of all children's
union, but this doesn't mean that the childrens' positions relative to
each other have not changed.

Instead, invalidate the entire projection to at least avoid constant
reallocation of the tile pyramid.

Found by group layer testing hero Tobias Jakobs.

(will have to optimize real translations of the entire group (or of the
only child in the group) differently)

app/core/gimpgrouplayer.c

@@ -869,6 +869,14 @@ gimp_group_layer_update_size (GimpGroupLayer *group)
       else
         {
           gimp_item_set_offset (item, x, y);
+
+          /*  invalidate the entire projection since the poition of
+           *  the children relative to each other might have changed
+           *  in a way that happens to leave the group's width and
+           *  height the same
+           */
+          gimp_projectable_invalidate (GIMP_PROJECTABLE (group),
+                                       x, y, width, height);
         }
 
       if (group->offset_node)