Properly count the number of bytes copied over from buffer in the scroll-left case

Also fix the conditional determining when we can re-use buffered data in the scroll-left case

IGV-2055
Bug fix:  sort by sample attribute not working for variant tracks.  Fix submitted as git pull request:   https://github.com/broadinstitute/IGV/pull/70.

Also, some minor cleanup of TrackLoader and TribbleFeatureSource.
Merge remote-tracking branch: upstream/master