pcbnew: Fix polygon hittest

Previous version had minimum width to account for it being difficult to
click on small lines.  This should not be used for filled polygons

Fixes: lp:1808141
* https://bugs.launchpad.net/kicad/+bug/1808141

pcbnew/class_drawsegment.cpp

@@ -640,17 +640,15 @@ bool DRAWSEGMENT::HitTest( const wxPoint& aPosition ) const
 
     case S_POLYGON:
         {
-            #define MAX_DIST_IN_MM 0.25
-            int distmax = std::max( m_Width, Millimeter2iu( MAX_DIST_IN_MM ) );
-
             if( !IsPolygonFilled() )
             {
                 SHAPE_POLY_SET::VERTEX_INDEX i;
                 auto poly = m_Poly;  //todo: Fix CollideEdge to be const
-                return poly.CollideEdge(VECTOR2I( aPosition ), i, distmax );
+                return poly.CollideEdge(VECTOR2I( aPosition ), i,
+                        std::max( m_Width / 2, Millimeter2iu( 0.25 ) ) );
             }
             else
-                return m_Poly.Collide( VECTOR2I( aPosition ), distmax );
+                return m_Poly.Collide( VECTOR2I( aPosition ), m_Width / 2 );
         }
         break;