[cairo] cairo / xlib not updating window content

[Pavel Petrovic]

for records, also adding a solution from Uli Schlachter with a similar
surface that is painted to real surface
after each drawing, which may be more efficient than push/pop group (?):

  https://stackoverflow.com/questions/51569047/cairo-xlib-not-updating-window-content
  https://kempelen.dai.fmph.uniba.sk/test_cairo/test_cairo_Uli_Schlachter.c

thx, p.