struct ViewParameters

The parameters of the associated view and viewport, sufficient to correctly

interpret the position and scale of pointer events dispatched to this view.

Ordering. These parameters arrive over the same channel as pointer events,

to provide synchronous context for interpreting the position of pointer

events in the view's local coordinate system.

Inter-protocol redundancy. Some of these parameters may also be sent over an

independent channel dedicated to view control; the client is responsible for

correct use of asynchronously received parameters.

TODO(https://fxbug.dev/42162292): Rename viewport, it is used in Flatland.