heatmapgl box-select doesn't follow cursor outside the plot bounds

[ssfrr]

If you want to select a corner of a heatmapgl plot, you have to drag your cursor exactly to the corner. If you try to go down-then-left, or left-then-down, the zoom box will stop following when you go past the edge. Compare the behavior when selecting a corner of the normal heatmap type.

Example:
https://jsfiddle.net/xfj1yxyu/4/

[etpinard]

That's true of all gl2d traces. Merging with #130