[css] reduce the amount of duplicated code in selection matching
[css] reduce the amount of duplicated code in selection matching

Always pass the stylable into the selection match function, rather than
extracting the required values beforehand.

pwithnall/mx,jonnylamb/mx,clutter-project/mx,jonnylamb/mx,pwithnall/mx,clutter-project/mx,clutter-project/mx,jonnylamb/mx,pwithnall/mx,jonnylamb/mx,pwithnall/mx,clutter-project/mx