DEFEATURE: remove alpha-field support (partly broken) from gltf output

- when used with *any* alphaField and normalised (the usual case)
  would largely give a 0-1 corresponding to the min/max of the first
  component, but could also yield negative values.

- if the alpha field corresponds identically to colour field, it is
  readily possible to combine as into RGBA sequences. However, if the
  fields are different it potentially means referencing an opacity
  field that has not yet been sampled.  This impedes using the format
  for a streaming sampler without additional overhead and/or rewriting
  the alpha channel later.