Send requested size in all configure events

Summary:
Previously when updating the state we would send a configure event with
the size 0,0. This means the client chooses the size. For regular window
this works as most toolkits treat this to mean the size that we
previously requested. It's not explicit in the spec either way.

For maximised windows it's a problem, the spec clearly states that when
maximised clients must follow the size given. Telling the client to be
0,0 doesn't make sense.

By always sending our last requested size we remove any ambiguity.

Test Plan:
Ran
Maximised some windows and changed focus
WAYLAND_DEBUG showed we weren't sending a configure with 0,0
after startup

Reviewers: #kwin, zzag

Reviewed By: #kwin, zzag

Subscribers: zzag, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D16055