Update gnustep-back to 0.22.0

Noteworthy changes in version `0.22.0'
======================================

Added new user defaults to make app icons and mini windows sticky
(omnipresent)

Many improvements to Cairo rendering, including buffering, glyphs and
DPI scaling.

Noteworthy changes in version `0.20.0'
======================================

Cairo is now the default backend, but falls back to art and xlib
gracefully.

  Fixed drawing and flipping (goes along with gui change).

  More Windows improvements.

(rh)