Handle drawable/client destruction in pending swaps/flips

A pending swap or flip holds references to drawables and clients which
become invalid when destroyed. Add suitable resources to the database
to track those lifetimes and clean up the pending data structure then.

Later, when the pending swap or flip occurs, handle a missing drawable
by just discarding the flip or swap. Handle a missing client by not
sending an event or reply.

Signed-off-by: Keith Packard <keithp@keithp.com>

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