Float position values in drag input filter

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