[server] Don't make assumptions about the order of damage_buffer and attach requests
Summary:
The spec says nothing about the order between damage_buffer and attach
requests.
Test Plan: Firefox doesn't become frozen. Although there are still issues with resizing.
Reviewers: KWin, davidedmundson
Reviewed By: KWin, davidedmundson
Subscribers: davidedmundson, kde-frameworks-devel
Tags: Frameworks
Differential Revision: https://phabricator.kde.org/D28147