Make shadows work for windows 100% width or height

Summary:
Clients provide shadows as 9 images, which then get split into relevant quads.
To work out the full width we need the size of an item on the left, the middle and the right.

Some elements can be legitiamtely missing so to work out the width of the left edge we need to consider
all 3 of topleft,left,bottomleft.

Currently we checked both corners in case one was missing, but it's legitimate for both to be missing if the shadow
stretches the whole way.

BUG: 380825

Test Plan: Shadow on panel, rest looks the same

Reviewers: Plasma, mart

Reviewed By: Plasma, mart

Subscribers: graesslin, mart, plasma-devel, kwin, KWin

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D6164