[thredds] wms GetLegendGraphic background / format options

[Ivan PRICE <ivan.price@xxxxxxxxxxx>]

Hi there..

the GetLegendGraphic function is really good and super fast.. but the 
image always comes back the same size with a black background, i'm 
wondering if it is possible to configure the formatting of the legend at 
all in the request to make it suit our application a bit better.

cheers,

-ivan