Layers lost when changing basemap

[mvl22]

This is a long-standing problem with Mapbox GL JS -based sites where a basemap change means that layers are not reliably reloaded.

We have a workaround in place, but it is clearly not being effective, so needs some reworking.

[mvl22]

This posting helpfully summarises the workaround needed - basically the entire state has to be cached, and rebuilt on style change:

mapbox/mapbox-gl-js#4006 (comment)

In particular I have found that the source itself is lost, not just the data state.

[mvl22]

Now fixed.

This is not a 100% fix but basically works most of the time now.

This is difficult to resolve properly until Mapbox fix basemap layer changing timing problems upstream.