Fixed detaching a buffer

- Quite a few buffers weren't being checked properly when a buffer
  was getting detached from the current context.
- Only current objects (TransformFeedback, VertexArray) should be
  affected by this.

Change-Id: I621c9fa45a951db5634616884cf57d5cb21d9bda
Reviewed-on: https://swiftshader-review.googlesource.com/4748
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>