Summary:
A drag start request should be dismissed when the client does not have an
implicit pointer grab or the currently focused pointer surface is not the
origin. The conditions for that were wrong in the past.

Also just ignore the request and not post directly an error, that potentially
kills the client since by concurrency the client might have send a valid
request, that got invalidated through grab or focus change at the same time
on the server side.

Test Plan: Manually and autotest.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: adridg, kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D15072