Moved the generic Transform Feedback buffer out of the local Transform
Feedback Object (TFO), and into global state.

Bug chromium:818098
Tests: dEQP-GLES3.functional.state_query.indexed.transform_feedback_switching_buffer
Change-id: Ife86cc82bd57c33b9daec4870610b5eaea5c2f52
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/28436
Presubmit-Ready: Sean Risser <srisser@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Sean Risser <srisser@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>