MD2D model viewOptions: add viewPort parameters

[#47182753]

The following viewPort parameters have been added to
the MD2D model viewOptions:

    "viewPortWidth": <md2d-length>, 
    "viewPortHeight": <md2d-length>, 
    "viewPortX": <md2d-length>, 
    "viewPortY": <md2d-length>, 

No use is made of these yet.