Add ability to get the original vertex index from the convex hull (#3018)

This makes it possible to know which input verticies were picked to
to define the hull.

With this information it is much easier to map certain features from the
input mesh/point cloud to the output convex hull.

src/LinearMath/btConvexHullComputer.cpp

@@ -2673,6 +2673,7 @@ btScalar btConvexHullComputer::compute(const void* coords, bool doubleCoords, in
 	}
 
 	vertices.resize(0);
+	original_vertex_index.resize(0);
 	edges.resize(0);
 	faces.resize(0);
 
@@ -2683,6 +2684,7 @@ btScalar btConvexHullComputer::compute(const void* coords, bool doubleCoords, in
 	{
 		btConvexHullInternal::Vertex* v = oldVertices[copied];
 		vertices.push_back(hull.getCoordinates(v));
+		original_vertex_index.push_back(v->point.index);
 		btConvexHullInternal::Edge* firstEdge = v->edges;
 		if (firstEdge)
 		{

src/LinearMath/btConvexHullComputer.h

@@ -66,6 +66,9 @@ public:
 	// Vertices of the output hull
 	btAlignedObjectArray<btVector3> vertices;
 
+	// The original vertex index in the input coords array
+	btAlignedObjectArray<int> original_vertex_index;
+
 	// Edges of the output hull
 	btAlignedObjectArray<Edge> edges;