meta-surface-actor: Fix is_argb32 for unredirected windows

meta_surface_actor_is_argb32 assumes that lack of stex means that a window is
ARGB32. When we unredirect a window we detach the texture so we end up without
a texture. Given that should_unredirect returns FALSE when a window is argb32,
we know that this window is indeed not ARGB32.

Returing TRUE in that case causes us to flip between redirected and
unredirected on every paint.

So fix that by returning FALSE in that case.