Issue #2664: add a tooltip to "better compression" checkbox.

Zlib is a "better" compression in the meaning that it is a more advanced
and complex algorithm than RLE. And in most cases, it should end up in
smaller file sizes. But as any algorithm, there may be cases when the
expectations are not met (worst cases or such). That's the nature of the
maths. Still we should not make the checkbox text over-complicated (it
is not the place to teach algorithmic), yet we can at least add a small
tooltip text.

app/widgets/gimpsavedialog.c

@@ -324,6 +324,9 @@ gimp_save_dialog_add_extra_widgets (GimpSaveDialog *dialog)
   /* Compression toggle. */
   compression_toggle =
     gtk_check_button_new_with_label (_("Save this XCF file with better but slower compression"));
+  gtk_widget_set_tooltip_text (compression_toggle,
+                               _("On edge cases, better compression algorithms might still "
+                                 "end up on bigger file size; manual check recommended"));
 
   dialog->compression_frame = gimp_frame_new (NULL);
   gtk_frame_set_label_widget (GTK_FRAME (dialog->compression_frame), compression_toggle);