Method for getting the filters from the underlying query
Very hacky way of getting snippets to 'work' as I would 'expect'. If you snippet on a query like 'corpus:thing' then the returned snippet is pretty much nonsense for all fields. This gets round that by getting all the string values used in filters like that and uses them plainly (without the filter part) to snippet all the desired fields with. This will by no means be what everyone expects to happen though