Color attachment 0 (or any color attachments!) may not exist;
rasterizer behavior is supposed to be completely independent of this.

Bug: b/126417154
Change-Id: If16307a7964c267af254e2a62822ac38cf58c48c
Reviewed-on: https://swiftshader-review.googlesource.com/c/25809Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Chris Forbes <chrisforbes@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>