Show the right tooltip for the hover cell for Heatmap charts

[tarekquao]

This pull request addresses the issue https://github.com/finos/perspective/issues/2340 where the heatmap chart showed tooltips for the wrong cell; specifically, the nearest cell to the right when the cursor moves halfway in the "right" direction, instead of the actual cell currently being highlighted.

[texodus]

Thanks for the PR!

Can you please squash 7ebb076 and 5f9f7a3? Alternatively I can cherry-pick 7f1fef0 into a cleanup PR preserving attribution.