[R] Background color with jpeg and png devices

[Trent Piepho]

If I change the background color of a plot, with par(bg="red") for example, I
get a plot with a red background when I use the postscript or X11 devices. 
With the png or jpeg devices, the background is white. 

Is this the intended behavior, or a bug?  Is there anyway to get a non-white
background with the png device?  I'm using R-1.1.0 on Linux.

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