Refactor `VertexBufferBinding` to be safe
Refactor `VertexBufferBinding` to be safe

VertexBufferBinding` no longer has a mutable reference to a `VertexBuffer` (there is no safe way to get both a reference to both the `VertexBuffer` and the `ArrayBufferBinding` for the internal buffer at the same time). Instead, it has a reference to the counter within the `VertexBuffer`, plus a `PhantomData`

kylewlacy/glitter,kylewlacy/glitter