A few blend related context members were changed from scalars to
arrays in order to support independent blend.

Bug b/140193782

Change-Id: I5ca1153e952fe0d3899f68dc6cd7cc5d8a244b72
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/34113Tested-by: Alexis Hétu <sugoi@google.com>
Presubmit-Ready: Alexis Hétu <sugoi@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>