Split fills

I am not entirely happy with this, since it seems overly complicated, and even I am not quite sure I believe it is really correct.  Still, it does help squish a tricky bug (https://github.com/diagrams/diagrams-svg/issues/43) which is actually a problem for some users (https://github.com/timbod7/haskell-chart/issues/19), and passes the tests I could think of (see `test/SplitAttr.hs`).

See also the discussion at https://github.com/diagrams/diagrams-lib/pull/141 .