Embedding plot: Mouse over line misses hover or selection of point

[PaulHax]

Steps

1. Select images and apply transform (blur).
2. Hover mouse hover point and line.
3. Bug: Lose hover effect on point. Clicking de-selects images, even though mouse is over point.

When mouse is over line, we get onHover(point) callback with point === null.

PAIR-code/scatter-gl#137