Avoid collisions in generated variable usage.
Avoid collisions in generated variable usage.

* generator/CallbackGen.cs: s/result/__result/ for temporary callback
return values in the native invoker generation.

openmedicus/gtk-sharp,Gankov/gtk-sharp,akrisiun/gtk-sharp,sillsdev/gtk-sharp,sillsdev/gtk-sharp,antoniusriha/gtk-sharp,Gankov/gtk-sharp,sillsdev/gtk-sharp,akrisiun/gtk-sharp,openmedicus/gtk-sharp,sillsdev/gtk-sharp,Gankov/gtk-sharp,akrisiun/gtk-sharp,akrisiun/gtk-sharp,orion75/gtk-sharp,Gankov/gtk-sharp,openmedicus/gtk-sharp,openmedicus/gtk-sharp,antoniusriha/gtk-sharp,Gankov/gtk-sharp,antoniusriha/gtk-sharp,orion75/gtk-sharp,openmedicus/gtk-sharp,Gankov/gtk-sharp,orion75/gtk-sharp,akrisiun/gtk-sharp,antoniusriha/gtk-sharp,antoniusriha/gtk-sharp,openmedicus/gtk-sharp,openmedicus/gtk-sharp,sillsdev/gtk-sharp,orion75/gtk-sharp,orion75/gtk-sharp