compositor: Always assign an output when updating a surface transform

compositor: Always assign an output when updating a surface transform

Also make all the callers of weston_surface_assign_output() update the
transform instead. This makes sure that when the surface is assigned an
output its bouding box is valid.

This fixes a bug where a newly created surface would have a NULL output
assigned. This would cause weston_surface_schedule_repaint() to not
schedule a repaint, preventing the surface to be shown until something
else caused a repaint.