extend viewbox to the plotting area

[jordansread]

The current code for viewbox_limits seems to return the limits of the bbox in proj space after the transform - it does not increase the limits to account for the aspect ratio of the plot being different from the aspect ratio of the bbox.

So, things look like this:

where the rectangle in black is the viewbox_limits, but it should actually extend to the full bounds of the plot in this case (since the plot was created w/ the same dimensions of the plotting metadata).