Hover on sequence callback

[PaulHax]

When the mouse moves over a line in a sequence, onHover is called with null. The mouse is still over the point, but also on the sequence/line too.

Problems:

• When user clicks, they want to select the point, but scatter-gl does not think mouse is on a point.
• The point hover color change effect in my app flickers strangely as the mouse crosses over the line embedded in a point.