[gradsusr] Grads 2.1.a3 - Problem plotting Intel graphic

[Graziano Giuliani]

Done some play around in the code.

Setting backing_store in calling XChangeWindowAttributes to NotUseful 
(the default), get some change in the window plotting. I can get the 
plot display on clicking on window if gxout is contour or grfill.
Still some artifacts with gxout shaded.
Do Xlib and cairo agree on how to use backing store?

G.