finalize multipolygon rendering

handle multipolygon outlines analogously to untagged way-nodes if
they do not contain tags other than the mp-relation (for outer
lines) or do not contain interesting tags at all.

this also fixes the incorrect duplicate polygon rendering in the
case where an outer line contains polygon-like tags that should
only be tagged on the multipolygon

see #22
update tests and add a rather complex multipolygon test scenario
drop mp_outline flags from mapcss and exported geojson

see #22
improve simple polygon test case
fix bug with simple multipolygon rendering
harmonize colouring of simple/normal multipolygons:

show them all in the "relation member" colour, for consistency.