Bug 588512 - Grid Spacing errors depending on unit

Always round grid spacings and offsets to integer precision when
using them for snapping and grid drawing. While we might *want* to
have subpixel precision grids, all the UI is pretending it to be
on pixel boundaries. Make that pretense an fact.

(cherry picked from commit 08c6aefa2398dcd36f0c8963633715de6a6430b4)