Heatmap zsmooth=fast bug

[lomereiter]

Seems that this line is the offending one, n instead of imageWidth should solve the problem, though I'm too lazy to check that.

Compare zmooth='fast' vs 'best'/false here: https://jsfiddle.net/hnbyn502/

[etpinard]

This indeed looks like a bug. Thanks for the report!

Moreover, I've just noticed that we don't have any image test mocks for zsmooth: 'fast':