Summary:
We lost information when using QMouseEvent::globalPos, since
it is integer. Use instead InputRedirection::globalPointer,
which is updated before the filters are processed and is
float.

Test Plan: Manually.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: anthonyfieroni, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D15502