Add overlays for anchors, just like segments, for hovered/selected vector layers

[Keavon]

When a vector layer is hovered or selected (with the Select tool), we see blue overlays visualizations showing the segments of the geometry. But we don't have that for free-floating anchor points. That should be added, drawing each anchor as a small circle (roughly the size of a pixel). It shouldn't be drawn for anchors that are connected to at least one segment.