heatmap ordering bug

[alexcjohnson]

Heatmaps have a similar issue to one that was fixed for contour maps in #2891 - ordering gets messed up if you hide and reshow traces. See https://codepen.io/alexcjohnson/pen/OwYrKg - initially electric is in front as it should be:

But if you click "Hide & Show Trace 0" then Greys is in front:

"Hide & Show Trace 1" will bring electric back to the front.

See 597a845#r209345643 - Should be just a matter of replicating 597a845 for heatmap - and probably for carpet too, though it's probably unintended anyway if carpets overlap each other!