Offset by distance

Added Billboard.pixelOffsetScaleByDistance, which applies a scale to pixelOffset based on viewer distance.  Added to Label, DynamicBillboard, and DynamicLabel.  Sandcastle examples and unit tests have been updated.

Of note, I made a minor change to Label glyph positioning; added a private member, Billboard._translate for holding the glyph vertex translations.  This separation of pixelOffset from the glyph vertex translation was necessary to ensure that pixelOffsetScale is only applied to the pixelOffset, leaving the glyph vertex translations in tact.