Fix draw buffers support. Bug 19353282 Change-Id: I6d1022c42a3347b8deb01af15078769d794050cb Reviewed-on: https://swiftshader-review.googlesource.com/5146 Tested-by: Nicolas Capens <capn@google.com> Reviewed-by: Alexis Hétu <sugoi@google.com> Reviewed-by: Nicolas Capens <capn@google.com>