Changing ref properties must unlink existing refs

[jfirebaugh]

"Ref properties" are 'type', 'source', 'source-layer', 'minzoom', 'maxzoom', 'filter', and 'layout'. Of these, the first three are read-only (except see #7570 for a proposal to make 'source' read-write).

Change any of these properties must at minimum unlink layers that were originally linked via the "ref" mechanism, such that they act independently. Ideally, new possible refs are autodetected -- that's #6900. But short of that, this issue tracks the fact that the existing runtime style API will not behave as expected if you modify, say, filter or layout properties of a layer that was originally a ref.