Correct the sign of the area calculation. The front facing determination
can then be written to exactly match the spec -- CCW means positive area
is front facing.

Change-Id: Ibed3e26b0cf77030325044c7dc9e818264a8750d
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/26009Tested-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>