Use better name for geometryBeforeUpdateBlocking

Summary:
Rename geometryBeforeUpdateBlocking in order to better reflect that it
corresponds to the last frame geometry.

Test Plan: Compiles.

Reviewers: #kwin, romangg

Reviewed By: #kwin, romangg

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D24658