m_shapes now has two possible indices. The first one is populated if
the point is associated with an arc and the second index is populated
if the point is shared between two arcs.
This moves EESchema DLIST structures to rtree. These changes are more
fundamental than the pcbnew changes from 9163ac543888c01d11d1877d7c1
and 961b22d60 as eeschema operations were more dependent on passing
drawing list references around with SCH_ITEM* objects.