core, display: Enable resize for GUI layer copying

Resolves #12428 (for now)

As noted by Thomas Manni, GimpDrawable
does not initialize its push_resize_undo value
except via gimp_drawable_new ().
If the drawable is created in some other way,
it defaults to FALSE and does not save the
original size in history when resized or cropped.

Thomas proposed initializing it to TRUE in
the init () function. This is likely the correct
approach, but could cause some new bugs
right before the 3.0 release.

For now, this patch calls 
gimp_drawable_enable_resize_undo () for
GUI-specific operations (like opening as
layers or dragging and dropping layers) to
fix the regression for 3.0. Afterwards, we
can spend time confirming the init ()
approach and switch over to that.

app/core/gimpimage.c

@@ -5497,6 +5497,7 @@ gimp_image_add_layers (GimpImage   *image,
 
       gimp_image_add_layer (image, GIMP_LAYER (new_item),
                             parent, position, TRUE);
+      gimp_drawable_enable_resize_undo (GIMP_DRAWABLE (new_item));
       position++;
     }
 

app/display/gimpdisplayshell-dnd.c

@@ -292,6 +292,7 @@ gimp_display_shell_drop_drawable (GtkWidget    *widget,
 
       gimp_image_add_layer (image, new_layer,
                             GIMP_IMAGE_ACTIVE_PARENT, -1, TRUE);
+      gimp_drawable_enable_resize_undo (GIMP_DRAWABLE (new_layer));
 
       gimp_image_undo_group_end (image);