xdg-shell: update shsurf->output when maximizing

xdg-shell: update shsurf->output when maximizing

shsurf->output is the output that user expects the shell surface
is on. When maximizing, we don't have any explicit setting of the output
like in the case of fullscreening, so set the output to the one that
the surface is currently on. In the case that the surface is not mapped yet,
(if it ever happens) use the same heuristics as for fullscreening.

This fixes the size sent with configure event, when maximizing a window.
The size is now picked up by the correct output, but the maximized
window position is still wrong. [Pekka Paalanen]

Bug: 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>