Comment 28 for bug 1692394

Created attachment 372570
osdWindow: change the centering calculation

The current method relies on Clutter.ActorAlign.CENTER.
But it doesn't take into account the new size of the OSD, and
another calculation has to be made to correct it.
When the level bar is wide enough, a jitter of 1px can appear
from the difference between the two centering methods.

This moves the centering calculation from the screen actor to the
constraint box of the OSD.