Fixes:
* Wrong (off-by-one) import of specular hardness (aka specular exponent in OBJ).
* Bad usage of world color when exporting ambient color (though it seems to make sense
  on first look, this is bad because impossible to 'undo' on import - merging
  data external to object itself).
* Bad default values for diff/spec intensity in imported materials (OBJ does not
  have those, so we must assume they are 1.0).

Thanks to Luke Brookes (propuke) for finding all those glitches! :)