Git issue 870

Orphaned transcript parts (e.g. exon, CDS) were not being drawn. The referenced GFF should result in 4 features being drawn. 
chr1	internal	exon	1000	1100	.	+	1	ID=feature_with_type_exon;name=Exon
chr1	internal	CDS	1100	1200	.	+	1	ID=feature_with_type_CDS;name=CDS
chr1	internal	contig	1200	1300	.	+	1	ID=feature_with_type_contig;name=Contig
chr1	internal	gene	1300	1400	.	+	1	ID=feature_with_type_gene;name=Gene