Better region markings

[adamnovak]

This improves(?) the requested region marker so you can actually see the between-base locations, and runs a bar along the selected region.

Here's ref:0-2 in cactus.vg

We have two yellow triangles at the inter-base locations marking the bounds of the requested 0-based end-exclusive region, and a yellow line connecting them. They're also a slightly easier to see yellow than the circles were.

The region indicator, previously and now, matches the BED convention of 0-based, end-exclusive coordinates. But vg is still using 0-based, end-inclusive coordinates to fetch the graph.

I've also restored the scale bar for the Lancet examples, and anything else generated before vg's real subpath support, fixing a regression I made in 3aa66dd. But the Lancet BEDs are non-standardly 1-based (and I think also end-inclusive?), and the provided graphs are describing end-inclusive regions, so when looking at the Lancet examples the coordinates are not using the same convention as usual, and so the region amrker ends a base before the displayed graph.

We can sort out the varying coordinate conventions in a proper solution for #488, which isn't this.