desktop_shell.panel_size was only used to determine if sway should
rearrange the output when rendering the panel in the output_pre_render
hook. This is not needed since the output will have been arranged at
that point.
It also caused sway to rearrange all the time when running with two
or more different monitors/resolutions because panel_size kept changing
with every output_pre_render callback.
Should fix#514
f63512480c