Add hasSwapEvent getter
Summary:
Add a small getter to query information internally if the backend supports
swap events. Defaults to true as it is the default in the GBM Wayland backend.
Test Plan: i915
Reviewers: KWin
Subscribers: kwin
Tags: KWin
Maniphest Tasks: T11071
Differential Revision: https://phabricator.kde.org/D25298