bg=True freaks out on OSMesa builds

[danlipsa]

See:

#117

[durack1]

@danlipsa @doutriaux1 just for old times sake, this is related to #72, #71, #70, CDAT/cdat#1424, and CDAT/cdat#1397 (closed)

[doutriaux1]

tested on mac and linux with 8.1 it works for me.