This fixes two causes of segfaulting when an output is destroyed.
The first occurred when an output was never enabled. The issue was that
the destroy signal was never initialized so when it was emitted, sway
segfaulted. This was fixed by moving the initialization into
`output_create` since all outputs, regardless of whether they have ever
been enabled, will be destroyed at some point.
The second occurred when the cursor was on an output that was being
destroyed. The sway output would have already been removed, but if there
are other outputs, a cursor rebase would still occur. Since the
wlr_output still existed and the sway output was destroyed, the cursor
could be over nothing, resulting in a segfault when trying to get the
sway output, which was destroyed.
8fd3f32c79