[chore] Avoid confusion in viewstate context

[igorDykhta]

MapViewStateContext sometimes stores complete mapState, sometimes only viewport. This changes it so that MapViewStateContext only keeps the viewport with the other props stripped out, and we merge it with mapState where it is needed.

Also, renaming getViewportFromMapState in map-state-updaters.ts into pickViewportPropsFromMapState to avoid confusion with another function with the same name.