Add modeSize getter

Summary:
The getter pixelSize rotates the size of the mode. In contrast modeSize just
directly returns the current mode size.

Test Plan: Compiles

Reviewers: #kwin, zzag

Reviewed By: #kwin, zzag

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D25901