Disable "feature" that apparently was attempting to switch from displaying features to density outside the visibility window.  This was never announced or documented, and in retrospect seems like a bad idea.  It was discovered to be the source of a subtile bug in which features would disappear when zooming out.
Refactor for clarity & remove unused class
VCF track enhancements for 1KG genotypes (visibility window calc,  treatment of vcf.list files)
Merge remote-tracking branch: upstream/master