Only commit XdgOutput::done if changed

Summary:
XdgOutput no-ops if one calls setLogicalSize(someSize) and someSize
matches the last sent size

However, as we have an explicit done signal, we currently end up sending
this regardless.

This patches tracks if we've made any changes to commit in the done
event.

CCBUG: 400987

Reviewers: KWin, romangg

Reviewed By: KWin, romangg

Subscribers: romangg, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D19255