Currently, a floating window that's been fullscreened can send us
xdg_toplevel::move, and we'll enter seatop_move_floating, which lets us
drag the surface around while it's fullscreen. We don't want
this--fullscreen surfaces should always be aligned to the screen--so add
the same check that seatop_default already does when entering this mode.
Tested with Weston's weston-fullscreen demo, which sends a move request
if you click anywhere on its surface.
0ee54a5243