[wayland] Fix window sizing when restoring a window that was initially fullscreen

Summary: When creating a surface and setting it as fullscreen before
attaching its buffer KWin does not know the original dimensions of the
surface and tries to use an invalid value when unsetting the fullscreen
flag. This patch fixes this by sending a configure with the size of 0,0
- which according to xdg-shell spec means that the client is requested
to set its size by itself.

Test Plan:
1. Create a fullscreen Wayland window.
2. Toggle fullscreen off.
3. KWin should send a configure event with size 0,0 instead of 1,1.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: zzag, graesslin, davidedmundson, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D16710