Removed inline function reset_viewports().

The viewport array will be rebuilt by tail recursion as soon as arguments are
depleted.
This is probably more accurate.

filter_name should nevertheless be correct because it fetches the first
property name.
Added blank spatial filter to filter_documents().

I don't see any way to do this more concisely and it does exactly what it's
supposed to do (as proven by console log):
- Divert spatial filters to _list/bbox etc. (this has yet to be written)
- do not collapse them with the rest,
- and if for instance 'time' is set from 2012 to 2011 and happens to come
  before 'intersects', it'll do the whole filter collapse and apply
  'intersects' after this is done. Because I'd rather apply 'intersects' on the
  smaller geometry set.
This keeps bbox code maintenance low.
Diverted spatial filter to a chain of requests.

Unfortunately it won't get any simpler than that: I need to post to
- _view/geojson to get the geometry to be checked against,
- _bbox-filter/bbox to get get geometries that are considered to be related,
- _bbox-filter/geojson to check considered geometries in full detail.