xdg-shell: fix maximizing in multi-head setup

Set the right position of maximized window. Up until now we ignored
output's "position" and were using only the working area
of output which is in output-relative coordinates. This led to
showing the maximized window always on the first output.

This, along with the previous patch, fixes
https://bugs.freedesktop.org/show_bug.cgi?id=82967



Signed-off-by: Marek Chalupa <mchqwerty@gmail.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>