`d` is used to drive some indexing math later.

Bug: b/127343247
Change-Id: Id9da208f5a7b678748f129f97de03f5bf04bcbe1
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/26128Tested-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>