iconview: Only compute item sizes in one place
iconview: Only compute item sizes in one place

Code was using different places to compute the icon size when layouting
and when doing size requests. This resulted in non-matching behavior in
obscure cases. And that lead to iconviews that were too small.

jigpu/gtk,davidgumberg/gtk,Lyude/gtk-,jadahl/gtk,jessevdk/gtk,ahodesuka/gtk,chergert/gtk,bratsche/gtk-,chergert/gtk,ebassi/gtk,jadahl/gtk,ahodesuka/gtk,davidgumberg/gtk,Lyude/gtk-,davidt/gtk,Adamovskiy/gtk,Distrotech/gtk2,davidgumberg/gtk,Distrotech/gtk2,alexlarsson/gtk,Sidnioulz/SandboxGtk,jessevdk/gtk,jessevdk/gtk,grubersjoe/adwaita,alexlarsson/gtk,alexlarsson/gtk,bratsche/gtk-,jessevdk/gtk,bratsche/gtk-,ahodesuka/gtk,jessevdk/gtk,Sidnioulz/SandboxGtk,ebassi/gtk,Distrotech/gtk2,Lyude/gtk-,grubersjoe/adwaita,davidt/gtk,Sidnioulz/SandboxGtk,davidt/gtk,jessevdk/gtk,bratsche/gtk-,Adamovskiy/gtk,bratsche/gtk-,chergert/gtk,jessevdk/gtk,jigpu/gtk,msteinert/gtk,msteinert/gtk,chergert/gtk,chergert/gtk,grubersjoe/adwaita,ebassi/gtk,ebassi/gtk,Adamovskiy/gtk,ahodesuka/gtk,chergert/gtk,Lyude/gtk-,ebassi/gtk,davidgumberg/gtk,jadahl/gtk,davidt/gtk,davidgumberg/gtk,Adamovskiy/gtk,Lyude/gtk-,ahodesuka/gtk,Distrotech/gtk2,grubersjoe/adwaita,jadahl/gtk,jigpu/gtk,jigpu/gtk,msteinert/gtk,jigpu/gtk,Adamovskiy/gtk,msteinert/gtk,grubersjoe/adwaita,Sidnioulz/SandboxGtk,davidgumberg/gtk,Sidnioulz/SandboxGtk,Lyude/gtk-,Lyude/gtk-,alexlarsson/gtk,chergert/gtk,ahodesuka/gtk,jadahl/gtk,Sidnioulz/SandboxGtk,davidgumberg/gtk,ebassi/gtk,jigpu/gtk,grubersjoe/adwaita,ahodesuka/gtk,grubersjoe/adwaita,alexlarsson/gtk,msteinert/gtk,jadahl/gtk,davidgumberg/gtk,alexlarsson/gtk,Distrotech/gtk2,alexlarsson/gtk,davidt/gtk,Distrotech/gtk2,Lyude/gtk-,grubersjoe/adwaita,Adamovskiy/gtk,alexlarsson/gtk,davidt/gtk,Adamovskiy/gtk,jadahl/gtk,Adamovskiy/gtk,jadahl/gtk,jigpu/gtk,jigpu/gtk,msteinert/gtk,chergert/gtk,ahodesuka/gtk,bratsche/gtk-