Some CPU architectures require all memory accesses to be naturally aligned.
We read polygon outline spans in pairs, so we need two underflow spans.

b/37478805

Change-Id: If74fd59654fb6fa56cbed93122b496b9e86dda06
Reviewed-on: https://swiftshader-review.googlesource.com/9528Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>