box-layout: Fix floating point truncation when calculating a child's size
box-layout: Fix floating point truncation when calculating a child's size

The child size is a float, not an int.

https://bugzilla.gnome.org/show_bug.cgi?id=707808

djdeath/clutter,ebassi/clutter,ebassi/clutter,ebassi/clutter,jigpu/clutter,jigpu/clutter,ebassi/clutter,djdeath/clutter,jigpu/clutter,djdeath/clutter,djdeath/clutter,Distrotech/clutter,ebassi/clutter,Distrotech/clutter,jigpu/clutter,jigpu/clutter,djdeath/clutter,GNOME/clutter,GNOME/clutter,Distrotech/clutter,Distrotech/clutter,GNOME/clutter,GNOME/clutter,djdeath/clutter,GNOME/clutter,jigpu/clutter,Distrotech/clutter,Distrotech/clutter