Fix of paint on supports don't work for object that has been scaled up #6718

The triangle-ray intersection function used a hard coded epsilon,
which did not work for triangle meshes, that were either too small
or too large. Newly the epsilon may be provided to the AABBTreeIndirect
search functions externally and IndexedMesh calculates a suitable
epsilon on demand from an average triangle mesh edge length.

src/libslic3r/TriangleMesh.cpp

@@ -1275,6 +1275,21 @@ float its_volume(const indexed_triangle_set &its)
     return volume;
 }
 
+float its_average_edge_length(const indexed_triangle_set &its)
+{
+    if (its.indices.empty())
+        return 0.f;
+
+    double edge_length = 0.f;
+    for (size_t i = 0; i < its.indices.size(); ++ i) {
+        const its_triangle v = its_triangle_vertices(its, i);
+        edge_length += (v[1] - v[0]).cast<double>().norm() + 
+                       (v[2] - v[0]).cast<double>().norm() +
+                       (v[1] - v[2]).cast<double>().norm();
+    }
+    return float(edge_length / (3 * its.indices.size()));
+}
+
 std::vector<indexed_triangle_set> its_split(const indexed_triangle_set &its)
 {
     return its_split<>(its);