Add ability to draw no main feature in glyphset_simple when the display is entirely composed of tags in rare cases.
Add ability to draw no main feature in glyphset_simple when the display is
entirely composed of tags in rare cases.

Use for new COSMIC lightning-bolt tracks.

Request from Laurent.

Ensembl/ensembl-webcode,Ensembl/ensembl-webcode,Ensembl/ensembl-webcode,Ensembl/ensembl-webcode,Ensembl/ensembl-webcode