Consider retaining pointer properties between actions

[jgraham]

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It's currently unclear what happens in actions when pointer property values are omitted. I think the implied behaviour is that if a value (say pressure) is omitted, we use the default values from https://www.w3.org/TR/pointerevents/#pointerevent-interface (in this case 1).

That means that if you perform a pointerDown and specify pressure: 0.5 each subssequent pointermove action on that pointer also needs to specify pressure: 0.5 or the pressure will be implictly increased to 1.

A more sensible approach would be to apply the defaults for pointerdown, but then keep the values in the input state and only update then for pointermove actions which explicitly supply a value.