Remove duplicate positions from polylines.

For #898.
I agree with @mramato.  Perhaps modify `PolylinePipeline.removeDuplicates` (it's private) so it also returns a boolean if duplicates were removed.  If they were, keep both the original and new version (as long as we need it to make the WebGL buffers).  If no duplicates were removed, `removeDuplicates` doesn't even need to create a new array.  Check other uses of the function to see if they were depending on it.