message: Fix a bug with duplicate location checking
  Before, if no location was specified multiple times (so set to -1),
  it would see that as a duplicate location.
diff: @@ -290,7 +290,7 @@ impl VertexArrayObject {
  for &(_, ref bindings, _, _, _) in vertex_buffers {
  for (i, bi) in bindings.iter().enumerate() {
  for (o, bo) in bindings.iter().enumerate() {
  - if i != o && bi.2 == bo.2 {
  + if i != o && bi.2 == bo.2 && bi.2 != -1 {
  panic!("The program attribute `{}` has the same binding location as program attribute `{}` (binding location {})",
  bi.0, bo.0, bi.2)
  }
  