Defer interface registration so all known interfaces can be called in
the order defined by the array of supported types.
Without this, the compositor defined the order of registration so it
wasn't possible to rely on registration functions to depend on other
interfaces.
This caused initialization for 'seats' to be moved out of the
register callback to ensure multiple interfaces were initialized.
This isn't good for readability or maintenance since it meant the
add/remove callbacks didn't act on matching data.
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