Flip arc endpoints when mirroring in FPEditor.

Fixes https://gitlab.com/kicad/code/kicad/issues/10550

(cherry picked from commit 7f6bc28f87)

pcbnew/fp_shape.cpp

@@ -305,6 +305,12 @@ void FP_SHAPE::Mirror( const wxPoint& aCentre, bool aMirrorAroundXAxis )
             MIRROR( m_bezierC2_0.x, aCentre.x );
         }
 
+        if( GetShape() == SHAPE_T::ARC )
+        {
+            std::swap( m_start, m_end );
+            std::swap( m_start0, m_end0 );
+        }
+
         if( GetShape() == SHAPE_T::BEZIER )
             RebuildBezierToSegmentsPointsList( m_width );