compositor: only reflow the outputs if the shell did not move them

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>