The BlendState structure is part of the cache key used to look up pixel
routines, which must not contain any uninitialized bits. The
PixelProcessor::States structure gets zero-initialized as a whole by
the Memset<> base class, but when writing BlendState structures into it
any uninitialized bits may get copied over.

Thus BlendState must also be zero-initialized, using Memset<>.

Bug: b/140286664
Bug: b/140193782
Change-Id: I031ec1a5f3be4329ae16366bfd4c8f17e2371384
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/35768Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>