Use more accurate hull shapes for round solids

Fixes https://gitlab.com/kicad/code/kicad/-/issues/8561

pcbnew/router/pns_solid.cpp

@@ -59,7 +59,7 @@ static const SHAPE_LINE_CHAIN buildHullForPrimitiveShape( const SHAPE* aShape, i
         return OctagonalHull( circle->GetCenter() - VECTOR2I( r, r ),
                               VECTOR2I( 2 * r, 2 * r ),
                               cl + 1,
-                              0.52 * ( r + cl ) );
+                              0.585 * ( r + cl ) );
     }
 
     case SH_SEGMENT:

pcbnew/router/pns_via.cpp

@@ -79,9 +79,10 @@ const SHAPE_LINE_CHAIN VIA::Hull( int aClearance, int aWalkaroundThickness, int
     if( !ROUTER::GetInstance()->GetInterface()->IsFlashedOnLayer( this, aLayer ) )
         width = m_drill;
 
-    return OctagonalHull( m_pos -
-        VECTOR2I( width / 2, width / 2 ), VECTOR2I( width, width ),
-        cl + 1, ( 2 * cl + width ) * 0.26 );
+    // Chamfer = width * ( 1 - sqrt(2)/2 ) for equilateral octagon
+    return OctagonalHull( m_pos - VECTOR2I( width / 2, width / 2 ),
+                         VECTOR2I( width, width ),
+                         cl + 1, ( 2 * cl + width ) * 0.2928 );
 }