Add support for VW property

[julianandrews]

The VW property specifies, as a list of points, the points to display. Ideally sgf-render would only render the points specified by VW (which could be overridden with an explicit --range option).

Since the points sets can be disconnected (example) this would be a large-ish change in the rendering though, so I'm not confident it's worth implementing.

The VW property is 'inherit', so we'd also need to keep track, as we step through the game of the current state of VW, but that should be easy enough. VW[] resets to no property.

More details here.