Using an initialization method or in-class initialization of each member
is bug-prone. Zero-initialization performed by the default constructor
guarantees repeatability.

Also don't skip drawing when the multisample mask is zero. Pipeline
stages prior to fragment processing may have side-effects.

Bug: b/155359026
Change-Id: Ibab131a41282fbdc4f5f84208776725eb3c786bd
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/45748
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>