- Remove Primitive::area, it is unused
- Always set front/back masks
- Initialize frontfacing based on front/back mask

Bug: b/118386749
Test: dEQP-VK.glsl.builtin_var.*
Change-Id: I8ccb5a2deb64df2dc4774a3511a5d30c09c30e5f
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/29208
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Chris Forbes <chrisforbes@google.com>
Presubmit-Ready: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>