For perspective camera, specifying zfar is not required

[michaliskambi]

Loading a model from https://poly.google.com/view/9Vq_-HEtIHA (downloaded version using "Updated GLTF file") raises an error

Invalid GLTF document, missing "zfar" field

Indeed, glTF file doesn't specify zfar:

   "cameras" : [
      {
         "perspective" : {
            "yfov" : 0.7853981633974483,
            "znear" : 0.1000000014901161
         },
         "type" : "perspective"
      }
   ],

Looking at glTF 2.0 specification, zfar is not required for perspective cameras: https://github.com/KhronosGroup/glTF/tree/master/specification/2.0#reference-perspective . It says "Required: No" and "If zfar is undefined, runtime must use infinite projection matrix." . But PasGLTF requires it, by reading it through Required function,

result.fPerspective.fZFar:=TPasJSON.GetNumber(Required(....

on https://github.com/BeRo1985/pasgltf/blob/master/src/PasGLTF.pas#L4050 .

Also aspectRatio is not required, according to the glTF 2.0 specification, but PasGLTF code makes it required.

Note that zfar is still required for orthographic cameras, which may explain why this bug appeared :)