shell: Remove confusing case fall-through
shell: Remove confusing case fall-through

This avoids calling weston_surface_set_position twice on the same surface. The
second call has no effect in many cases because sx and sy
are usually zero on this path.

This change now means that any sx/sy values passed into ::attach will be
ignored on the first attach for popup surfaces. This similar to the behaviour
for other surface types.

jonnylamb/weston,udoprog/weston,eyolfson/weston,eyolfson/weston,Tarnyko/weston-xdg_surface_present,giucam/weston,sir-murray/weston,mchalupa/weston,kwm81/weston,Fantu/compositor-spice,sir-murray/weston,krezovic/weston,jonnylamb/weston,udoprog/weston,kwm81/weston,krezovic/weston,Tarnyko/weston-xdg_surface_present,xorgy/weston,mchalupa/weston,Fantu/compositor-spice,Fantu/compositor-spice,giucam/weston,xorgy/weston,Gnurou/weston,Gnurou/weston