D10736: Do clamping in the model and add missing lower bound check

[Kai Uwe Broulik]

broulik accepted this revision.
This revision is now accepted and ready to land.

REPOSITORY
  R119 Plasma Desktop

BRANCH
  Plasma/5.12

REVISION DETAIL
  https://phabricator.kde.org/D10736

To: hein, #plasma, broulik
Cc: plasma-devel, ZrenBot, lesliezhai, ali-mohamed, jensreuterberg, abetts, 
sebas, apol, mart