patch for gtksheet

I noticed 2.1.1 gtksheet is freeing pixmaps with g_free(). For
example, in gtk_sheet_make_backing_pixmap() it has:

          g_free(sheet->pixmap);
          sheet->pixmap = gdk_pixmap_new (sheet->sheet_window,
                                               width, height,
                                               -1);

This actually works, bizarrely, on my 32-bit laptop, but crashes
horribly on a 64-bit desktop. I changed it to:

          g_object_unref(sheet->pixmap);

(there's another g_free(->pixmap) in gtk_sheet_unrealize () that also
needs changing) and all seems well now. I can run my app in valgrind
without problems.

John