Polygon triangulation: Check for broken remainders

If the last three points of a tesselation are concave, we will never be
able to triangulate them.  They were likely formed from a bad polygon,
so we will drop the triangle and return completed

Fixes https://gitlab.com/kicad/code/kicad/issues/9380

libs/kimath/include/geometry/polygon_triangulation.h

@@ -468,6 +468,15 @@ private:
             }
         }
 
+        // Check to see if we are left with only three points in the polygon
+        if( aPoint->next && aPoint->prev == aPoint->next->next )
+        {
+            // Three concave points will never be able to be triangulated because they were
+            // created by an intersecting polygon, so just drop them.
+            if( area( aPoint->prev, aPoint, aPoint->next ) >= 0 )
+                return true;
+        }
+
         /*
          * At this point, our polygon should be fully tessellated.
          */

libs/kimath/src/geometry/shape_poly_set.cpp

@@ -2378,6 +2378,10 @@ void SHAPE_POLY_SET::CacheTriangulation( bool aPartition )
 
     while( tmpSet.OutlineCount() > 0 )
     {
+
+        if( !m_triangulatedPolys.empty() && m_triangulatedPolys.back()->GetTriangleCount() == 0 )
+            m_triangulatedPolys.erase( m_triangulatedPolys.end() - 1 );
+
         m_triangulatedPolys.push_back( std::make_unique<TRIANGULATED_POLYGON>() );
         PolygonTriangulation tess( *m_triangulatedPolys.back() );