fix(layout): properly serialize the DataPanelLayout in the state
fix(layout): properly serialize the DataPanelLayout in the state

This corrects a oversight in 709e5778ff0f9e6cf6583f13656581aa3e05cb52, in that the DataPanelLayout
type was not properly serialized in the state if it was equal to the default value of "xy" and
neither crossSections nor orthographicProjection were set.

google/neuroglancer,google/neuroglancer,google/neuroglancer,google/neuroglancer,google/neuroglancer,google/neuroglancer,seung-lab/neuroglancer,seung-lab/neuroglancer,janelia-flyem/neuroglancer,janelia-flyem/neuroglancer,seung-lab/neuroglancer,seung-lab/neuroglancer,seung-lab/neuroglancer,seung-lab/neuroglancer,google/neuroglancer,google/neuroglancer,janelia-flyem/neuroglancer,janelia-flyem/neuroglancer,janelia-flyem/neuroglancer,seung-lab/neuroglancer