Fix opacity for clone brush engine

Summary:
https://bugs.kde.org/show_bug.cgi?id=380117

Clone brush ignore curves for opacity. This patch should fix the bug.

This opacity looks more like flow but I think it's still better than ignoring pressure (and other sensors) at all.

Reviewers: rempt, dkazakov

Reviewed By: rempt

Differential Revision: https://phabricator.kde.org/D6536