Summary:
So far when deleting a Shadow we used deleteLater which caused it
to be deleted in the next event cycle. This could in worst case result
in the Shadow being deleted after compositing got suspended. Thus the
Shadow not getting removed from the DecorationShadowCache which in
turn would mess up rendering on resume of compositing as the cache
returns a texture created for a different context.

BUG: 361154
FIXED-IN: 5.7.4

Reviewers: #kwin, #plasma

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D2483