desktop-shell: Fix shell fade animations

Shell fade per output was preperatory work for an implementation of the zwp_idle_inhibit protocol, and nobody ever followed up on that.

Now that we can have overlapping outputs, having a curtain per output isn't right, because we can have overlapping curtains, which look pretty weird.

Most importantly though, this fixes a nasty bug: If, on a multiple output system, an output is disconnected during the fade out and reconnected before an event causes a fade in, the fade in will result in a crash (by way of manually destroying the curtain, which causes the animation to be destroyed, which causes the curtain to be destroyed a second time.)

Signed-off-by: Derek Foreman derek.foreman@collabora.com