canvas.png writes a PNG with labels off if bg=0

[chaosphere2112]

Running this test
test2.txt
produces

Uncommenting the line that sets bg to 1 fixes the problem.

Migrated from: CDAT/cdat#1740

[danlipsa]

@chaosphere2112 @durack1 @doutriaux1 This works with vcs SHA f6a8867 from Apr 5.