weston_compositor_reflow_outputs() assumes that all output are positioned from
left to right with no gaps in the same order in which they where created.
If the shell moves an output with weston_output_move() then this assumption is
no longer true. So stop reflowing the outputs in the case. The shell is now
responsible for positioning all outputs as needed.
Signed-off-by: Michael Olbrich <m.olbrich@pengutronix.de>