router: pick similar starting shapes for the diff pairs, not the very nearest one

pcbnew/router/pns_diff_pair_placer.cpp

@@ -540,7 +540,14 @@ bool PNS_DIFF_PAIR_PLACER::findDpPrimitivePair( const VECTOR2I& aP, PNS_ITEM* aI
 
             double dist = ( *anchor - *refAnchor ).EuclideanNorm();
 
-            if( dist < bestDist )
+            bool shapeMatches = true;
+
+            if( item->OfKind( PNS_ITEM::SOLID ) && item->Layers() != aItem->Layers() )
+            {
+                shapeMatches = false;
+            }
+
+            if( dist < bestDist && shapeMatches )
             {
                 found = true;
                 bestDist = dist;

pcbnew/router/pns_layerset.h

@@ -111,6 +111,16 @@ public:
         return PNS_LAYERSET( 0, 256 ); // fixme: use layer IDs header
     }
 
+    bool operator==( const PNS_LAYERSET& aOther ) const
+    {
+        return ( m_start == aOther.m_start ) && ( m_end == aOther.m_end );
+    }
+
+    bool operator!=( const PNS_LAYERSET& aOther ) const
+    {
+        return ( m_start != aOther.m_start ) || ( m_end != aOther.m_end );
+    }
+
 private:
     int m_start;
     int m_end;