Position OSD a bit farther down

Summary:
A common user complains is that our OSDs--particularly the volume OSD--are too intrusive
and get in the way of the screen content. For example when adjusting the volume while
watching a full-screen video, the volume change OSD will typically appear right in the
middle of an actor's face.

D20569 was an attempt to use a horizontal OSD to alleviate this issue. It mostly worked,
but IMO it was still positioned too high up.

This patch moved the placement down a little bit to make the OSD appear even farther from
the center of the screen to make it less likely to

I tried not to move it down too far or else it would interfere with subtitles in videos.

Test Plan:
With D20569:

Reviewers: KWin, VDG, broulik, ndavis, zzag

Reviewed By: KWin, VDG, ndavis, zzag

Subscribers: meven, ndavis, niccolove, baberts, davidedmundson, filipf, zzag, kori, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D29263