PanelView: align setting of masks with how it's done for dialogs/tooltips

Summary:
With Plasma dialogs & tooltips the mask is only set on the widget itself
if compositing is off. Instead is the mask explicitly passed for the
KWindowEffects.
The recent changes to fix the blurbehind & contrast mask with panels
instead turned to always set the mask on the widget. For some reason
yet to be understood this can sometimes result in an offset by a pixel
between the mask used for painting and the mask used for the window effects.

Aligning the mask setting code with the one for dialogs & tooltips makes
the symptom go away for now, and also delivers consistency.

BBUG: 406380
FIXED-IN: 5.16.1

Test Plan:
Using different themes, including Adapta & Arc Dark, a gap can no longer
been seen. Only with compositing turned off, which is old behaviour due to
the real bug yet to understand completly.
Resizing panels or changing screen resolution also works without breaking
rendering or panel contents interaction.

Reviewers: #plasma, hein

Reviewed By: #plasma, hein

Subscribers: hein, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D21803