[ASTextNode] use accessor for `pointSizeScaleFactors`
[ASTextNode] use accessor for `pointSizeScaleFactors`

I have a subclass of `ASTextNode` that uses `pointSizeScaleFactors`. Currently I have to recompute the scale factors any time the font size changes. If `_rendererAttributes` used the property accesor for the scale factors I would only have to create them when asked. I hope this little change can make it in :)

bimawa/AsyncDisplayKit,flovouin/AsyncDisplayKit,smyrgl/AsyncDisplayKit,maicki/AsyncDisplayKit,Xinchi/AsyncDisplayKit,garrettmoon/AsyncDisplayKit,samhsiung/AsyncDisplayKit,george-gw/AsyncDisplayKit,tomizimobile/AsyncDisplayKit,JetZou/AsyncDisplayKit,gazreese/AsyncDisplayKit,george-gw/AsyncDisplayKit,tomizimobile/AsyncDisplayKit,rmls/AsyncDisplayKit,aaronschubert0/AsyncDisplayKit,lappp9/AsyncDisplayKit,gazreese/AsyncDisplayKit,george-gw/AsyncDisplayKit,aaronschubert0/AsyncDisplayKit,tomizimobile/AsyncDisplayKit,lappp9/AsyncDisplayKit,Xinchi/AsyncDisplayKit,romyilano/AsyncDisplayKit,rahul-malik/AsyncDisplayKit,aaronschubert0/AsyncDisplayKit,facebook/AsyncDisplayKit,harryworld/AsyncDisplayKit,garrettmoon/AsyncDisplayKit,Adlai-Holler/AsyncDisplayKit,facebook/AsyncDisplayKit,JetZou/AsyncDisplayKit,harryworld/AsyncDisplayKit,jellenbogen/AsyncDisplayKit,rahul-malik/AsyncDisplayKit,maicki/AsyncDisplayKit,romyilano/AsyncDisplayKit,samhsiung/AsyncDisplayKit,harryworld/AsyncDisplayKit,garrettmoon/AsyncDisplayKit,tomizimobile/AsyncDisplayKit,george-gw/AsyncDisplayKit,rmls/AsyncDisplayKit,samhsiung/AsyncDisplayKit,gazreese/AsyncDisplayKit,bimawa/AsyncDisplayKit,smyrgl/AsyncDisplayKit,flovouin/AsyncDisplayKit,rahul-malik/AsyncDisplayKit,Xinchi/AsyncDisplayKit,jellenbogen/AsyncDisplayKit,levi/AsyncDisplayKit,jellenbogen/AsyncDisplayKit,smyrgl/AsyncDisplayKit,rmls/AsyncDisplayKit,bimawa/AsyncDisplayKit,Adlai-Holler/AsyncDisplayKit,flovouin/AsyncDisplayKit,bimawa/AsyncDisplayKit,rmls/AsyncDisplayKit,aaronschubert0/AsyncDisplayKit,Xinchi/AsyncDisplayKit,JetZou/AsyncDisplayKit,maicki/AsyncDisplayKit,gazreese/AsyncDisplayKit,maicki/AsyncDisplayKit,Adlai-Holler/AsyncDisplayKit,chrisdanford/AsyncDisplayKit,facebook/AsyncDisplayKit,levi/AsyncDisplayKit,harryworld/AsyncDisplayKit,smyrgl/AsyncDisplayKit,chrisdanford/AsyncDisplayKit,levi/AsyncDisplayKit,facebook/AsyncDisplayKit,lappp9/AsyncDisplayKit,Xinchi/AsyncDisplayKit,Adlai-Holler/AsyncDisplayKit,tomizimobile/AsyncDisplayKit,maicki/AsyncDisplayKit,flovouin/AsyncDisplayKit,romyilano/AsyncDisplayKit,levi/AsyncDisplayKit,JetZou/AsyncDisplayKit,chrisdanford/AsyncDisplayKit,romyilano/AsyncDisplayKit,aaronschubert0/AsyncDisplayKit,jellenbogen/AsyncDisplayKit,samhsiung/AsyncDisplayKit,lappp9/AsyncDisplayKit,JetZou/AsyncDisplayKit,chrisdanford/AsyncDisplayKit,rmls/AsyncDisplayKit,harryworld/AsyncDisplayKit,jellenbogen/AsyncDisplayKit,garrettmoon/AsyncDisplayKit,Adlai-Holler/AsyncDisplayKit,garrettmoon/AsyncDisplayKit,rahul-malik/AsyncDisplayKit,levi/AsyncDisplayKit,flovouin/AsyncDisplayKit