Fix SHAPE_LINE_CHAIN::Simplify() when there are only 3 points in the chain

We weren't updating m_shapes properly.

Fixes https://gitlab.com/kicad/code/kicad/-/issues/11695

libs/kimath/src/geometry/shape_line_chain.cpp

@@ -1696,7 +1696,7 @@ SHAPE_LINE_CHAIN& SHAPE_LINE_CHAIN::Simplify( bool aRemoveColinear )
     else if( PointCount() == 3 )
     {
         if( m_points[0] == m_points[1] )
-            m_points.pop_back();
+            Remove( 1 );
 
         return *this;
     }

qa/unittests/libs/kimath/geometry/test_shape_line_chain.cpp

@@ -106,6 +106,7 @@ BOOST_AUTO_TEST_CASE( ArcToPolylineLargeCoords )
     base_chain.ClearArcs();
 
     BOOST_CHECK( GEOM_TEST::IsOutlineValid( base_chain ) );
+    BOOST_CHECK_EQUAL( base_chain.CPoints().size(), base_chain.CShapes().size() );
     BOOST_CHECK_EQUAL( base_chain.PointCount(), 11 ); // We should have the same number of points
     BOOST_CHECK_EQUAL( base_chain.ArcCount(), 0 ); // All arcs should have been removed
     BOOST_CHECK_EQUAL( base_chain.Area(), areaPriorToArcRemoval ); // Area should not have changed
@@ -130,10 +131,30 @@ BOOST_AUTO_TEST_CASE( SetClosedDuplicatePoint )
 
     // CLOSED CHAIN
     chain.SetClosed( true );
+    BOOST_CHECK_EQUAL( chain.CPoints().size(), chain.CShapes().size() );
     BOOST_CHECK_EQUAL( chain.PointCount(), 30 ); // (-1) should have removed coincident points
     BOOST_CHECK( GEOM_TEST::IsOutlineValid( chain ) );
 }
 
+// Test that duplicate point gets removed when we call simplify
+BOOST_AUTO_TEST_CASE( SimplifyDuplicatePoint )
+{
+    SHAPE_LINE_CHAIN chain;
+
+    chain.Append( { 100, 100 } );
+    chain.Append( { 100, 100 }, true ); //duplicate point to simplify
+    chain.Append( { 200, 100 } );
+
+    BOOST_CHECK( GEOM_TEST::IsOutlineValid( chain ) );
+    BOOST_CHECK_EQUAL( chain.PointCount(), 3 );
+
+    chain.Simplify();
+
+    BOOST_CHECK_EQUAL( chain.CPoints().size(), chain.CShapes().size() );
+    BOOST_CHECK_EQUAL( chain.PointCount(), 2 ); // (-1) should have removed coincident points
+    BOOST_CHECK( GEOM_TEST::IsOutlineValid( chain ) );
+}
+
 
 // Test special case where the last arc in the chain has a shared point with the first arc
 BOOST_AUTO_TEST_CASE( ArcWrappingToStartSharedPoints )