Updating 3d columns with z stacking fails

[TorsteinHonsi]

Demo: http://jsfiddle.net/highcharts/m69rbc9p/15/

The demo shows updating showInLegend, but it also applies to changing Y value etc.

[pawelfus]

I think this example shows better what happens: http://jsfiddle.net/m69rbc9p/16/ - all columns are placed with wrong groupZPadding.

[pawelfus]

Small debugging:

This is caused by this line - series._i is wrong (?) when series are updated because of this line. We can replace series._i with series.index, however, there will be still problem with zIndexes caused by those line. When removing this part we get working code - all tests are passed, but I'm not sure what is the purpose of this part..

[TorsteinHonsi]

Okay, please feel free to provide a fix. As for the lines in Column.js, they point to #4062 so at least you need to check that.

[TorsteinHonsi]

... and otherwise try to replicate the conditions of those lines and see what it does.

[pawelfus]

I didn't found any case, when it fails. There is only one thing I am concerned about: zIndexes are floating point numbers, not integers - we had similar issue with pie chart a while ago, but I don't want to add more logic if this is not necessary.

Additionally, I pushed test for #4062.