x.showbg() not implemented

[chaosphere2112]

we used to be able to plot in bg mode and at the end bring the canvas to the front.

import vcs
x=vcs.init()
x.plot([1,2,3,4,5,4,3,2,1],bg=1)  # plots in bg
x.showbg()  # would bring up the canvas

Migrated from: https://github.com/UV-CDAT/uvcdat/issues/1598