multiple gl2d subplots: only one is drawn at a time

[alexcjohnson]

Open gl2d_stacked_subplots - only one subplot is visible at a time. Whenever you hover over one, it appears and the others disappear. Seems to have come in with #2159 cc @dfcreative @etpinard

[etpinard]

Must be a side effect of #2159