Fix culling to match Vulkan convention

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/+/26009
Tested-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>

Author: chrisforbes

src/Pipeline/SetupRoutine.cpp

@@ -83,7 +83,7 @@ namespace sw
 				Float y1 = Float(Y[1]);
 				Float y2 = Float(Y[2]);
 
-				Float A = (y2 - y0) * x1 + (y1 - y2) * x0 + (y0 - y1) * x2;   // Area
+				Float A = (y0 - y2) * x1 + (y2 - y1) * x0 + (y1 - y0) * x2;   // Area
 
 				If(A == 0.0f)
 				{