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