[effects/screenedge] Support "hint-stretch-borders"

Summary:
Some old themes have the flag set and also a style which expects the
borders to be stretched. Given that the documentation on techbase
describes widgets/glowbar still as "a frame without a prefix", one also
would assume that all the optional hints (which make sense) still apply.
Even more, the Air & Oxygen themes have the hint also set, though for
their rendering it makes no difference.
The small code needed seems worth the unbreaking of old themes as well
as giving theme creators another variable of freedom for their styles.

Test Plan:
Glow bar still works on all corners and edges with all themes as before,
though rendering now as expected for themes which have the
"hint-stretch-borders" set.

Reviewers: #kwin, zzag, davidedmundson

Reviewed By: #kwin, zzag, davidedmundson

Subscribers: davidedmundson, zzag, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D20621