precalculate_sizes: round sizes instead of flooring them
precalculate_sizes: round sizes instead of flooring them

This will lead to more accurate and consistent container sizes.

Needed to fix the failing test of #3240.

i3/i3,stapelberg/i3,Airblader/i3-original,Airblader/i3-original,Airblader/i3,stapelberg/i3,Airblader/i3,Airblader/i3-original,i3/i3,Airblader/i3-original,Airblader/i3,i3/i3,Airblader/i3-original,stapelberg/i3,stapelberg/i3,i3/i3,Airblader/i3