cairo: Added SetSourceColor method to Cairo.Context.
cairo: Added SetSourceColor method to Cairo.Context.

Cherry-picked from the gtk-sharp-2.12 branch.

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