backends/wayland: Round passed scale

Converting to wl_fixed is lossy. Especially for non-round values like
1.15.

This causes various visual glitches and off sizes.
Given we're using units of 120ths for the fractional scale protocol it
makes sense to fix the passed values for anything else over the wayland
protocol.

CCBUG: 465850


(cherry picked from commit 3b7b173a)