[GeospatialCamera] Add overrideable surfaceCollision logic. Fix center recalculation

[georginahalpern]

• getCollisionOffset is applied to center and position simultaneously so that the orientation remains the same (yaw/pitch/radius)
• camera extension can override _getCollisionOffset if wanting to customize collision logic, or a camera user can update collisionOffset property in the afterCheckInputsObservable
• only recalculate center after a drag so that the center is not continually changing during a zoomToCursor movement (causing slight jitter)

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