Paint small progressbars correctly on Vista+

The animated glow always has a length of 120 but was previously drawn
with the clipping rect of the bar's actual size. For sizes smaller than
120 the native theme part would be clipped and the black gradient would
show.

Change-Id: Id81e39c405ef81ae08af0f64249568016944bdf1
Task-number: QTBUG-51266
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>