Ensure bad polys get simplified before triangulation

If the poly set is self-intersecting, the triangulation may be
problematic and the full set needs simplification before.  We don't do
this uniformly because the simplification can be a slowdown if it is not
needed

Fixes https://gitlab.com/kicad/code/kicad/issues/12806

libs/kimath/src/geometry/shape_poly_set.cpp

@@ -2710,6 +2710,9 @@ void SHAPE_POLY_SET::CacheTriangulation( bool aPartition )
                 bool triangulationValid = false;
                 int pass = 0;
 
+                if( polySet.IsSelfIntersecting() )
+                    polySet.Simplify( PM_FAST );
+
                 while( polySet.OutlineCount() > 0 )
                 {
                     if( !dest.empty() && dest.back()->GetTriangleCount() == 0 )