Scale billboards based on distance (Issue #93)

Issuing pull request for preliminary review.  Due to improper scaling of Text Billboards, Labels have this feature disabled by default.  I selected a default value that looks ok for our typical usage, but I'd be more than happy to have this feature disabled by default.  Finally, the storage container for the scaleByDistance member is a list of 4 floats.  Is it preferred to store this as a pair of Cartesian2 elements?