Fix bugs with label and billboard properties.

* Setting pixelOffset on a label did not always update the glyphs.  Fixes #1838.  Similar problem with vertical origin.
  * Previous tests changed all label properties at once, which triggered rebindAllGlyphs, hiding the problem.
* Fixed Billboard/Label crash when setting scaleByDistance/translucencyByDistance/pixelOffsetScaleByDistance to undefined.
  * Previous tests created billboard with undefined, then assigned undefined, which hid the problem.
* Minor Label performance improvements:
  * Setting pixelOffset does not need to repositionAllGlyphs, since it does not affect the glyph transforms.
  * translucencyByDistance/pixelOffsetScaleByDistance do not need to rebindAllGlyphs, they do not affect the glyph appearance or transforms.