Clear both cursor caches

Summary: There are two cursor caches: m_cursors and m_cursorsByName. The second doesn't ever seem to be cleared.

Reviewers: KWin, davidedmundson, graesslin

Reviewed By: KWin, graesslin

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D12542