struct TouchInteractionId

A unique identifier for a "interaction" of touch events in an event stream.

Touch events are observed as a succession of interactions, as fingers engage

and disengage with the display.

A finite sequence of pointer events that follows the `EventPhase` state

machine, starting from the initial state ADD, is called an **interaction**.

A closed (or past) interaction is one where it has reached the terminal

state (REMOVE or CANCEL); an open (or current) interaction is one where it

has not.

For a given device pointer, a stream of events is observed as a succession

of zero or more closed interactions (the past history of user engagement),

followed by at most one open interaction (the current user engagement).

Because we need to group pointer events by their interaction, touch event

carries an **interaction id** that is unique in that pointer stream. This

common reference makes it possible to operate on a closed interaction, as

well as an open interaction.

Also see `EventPhase` for a discussion on event streams by mice.