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