Default to white to replace transparency in JPEG export

By default, jpegs exported on a transparent background have the transparency converted to black. I can't imagine the reasoning for this, but since most of the time the canvas in Inkscape is white, it seems a good idea to default to white.

Here's a link to the issue and how to solve it programatically: https://stackoverflow.com/questions/50739732/how-to-covert-png-to-jpeg-using-pillow-while-image-color-is-black

@doctormo also gives the following hints:

"Just remember that in raster_export_jpg.py that self.img is the same thing as im in the above example."

"pars.add_argument("--fill_color", type=inkex.Color, default=inkex.Color(1364325887))"

"then you can just do list(self.option.fill_color.to('rgb')) which gives you to the RGB compatible with the above."