plugin: fix for errors during export of webp animation with file-webp plugin

This fixes an error during export of webp animations when the layer is size smaller than the image size.

https://bugzilla.gnome.org/show_bug.cgi?id=769872

plug-ins/file-webp/file-webp-save.c

@@ -387,6 +387,8 @@ save_animation (const gchar    *filename,
   if (nLayers < 1)
     return FALSE;
 
+  gimp_image_undo_freeze (image_ID);
+
   do
     {
       gint loop;
@@ -423,6 +425,9 @@ save_animation (const gchar    *filename,
           /* Obtain the drawable type */
           drawable_type = gimp_drawable_type (allLayers[loop]);
 
+          /* fix layers to avoid offset errors */
+          gimp_layer_resize_to_image_size (allLayers[loop]);
+
           /* Retrieve the buffer for the layer */
           geglbuffer = gimp_drawable_get_buffer (allLayers[loop]);
           extent = *gegl_buffer_get_extent (geglbuffer);