Instanced rendering only requires a
single attribute per instance, which
was handled properly everywhere except
in the streaming buffer, so that had to
be fixed. All related dEQP tests pass.

Change-Id: I9b9bd6cd790dfe7ecf7f5d2ed84623bfa4da4b6f
Reviewed-on: https://swiftshader-review.googlesource.com/3430Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Alexis Hétu <sugoi@google.com>