Removing a D3D limitation from SwiftShader

Removing a D3D limitation from SwiftShader

The limitation of not being able to have
different values for stencil and stencilBack
was put in place in Angle because of a D3D
limitation, which we do not have in Swift
Shader, so I doubt we need the same limitation.

FYI, this fixes about 600 dEQP tests.

Change-Id: I74d1786c612e817fb5ef46f233751e6f2581c33f
Reviewed-on: https://swiftshader-review.googlesource.com/3321
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>