GTK2: Fixed various issues in get_doc() et al.

* Changes API of pgtk2_get_doc(). The pushv parameter has been
  replaced with a pointer to the desired svalue position.

* get_doc() did not take advantage of pgtk2_get_doc() supporting
  pushing the result on the svalue stack.

* pgtk2_get_doc() returned an uninitialized pointer
  when called with a non-zero pushv.

* pgtk2_get_doc() returned a g_new0()'ed svalue when called with
  a zero pushv. This value was not freed later.
GTK2: Fixed infinite loop in encode_truecolor_24_rgb_al32().