Added a mask to prevent writing to unused outputs

Bug: b/162348737
Change-Id: I72b86cf2f6d0f9f07992cc63bc55fe9e45822e1d
Tests: dEQP-VK.draw.output_location.array.*
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/47168
Presubmit-Ready: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Alexis Hétu <sugoi@google.com>