simplify: Share quadric evaluation between position & attribute quadrics

The math for evaluating the error is the same up until normalization /
gradient accumulation differences, and since this code is sensitive wrt
performance / precision, it's nice to share it. This should not have any
observable performance impact if the compiler makes reasonable inlining
decisions.

src/simplifier.cpp

@@ -598,7 +598,7 @@ static void quadricAdd(QuadricGrad* G, const QuadricGrad* R, size_t attribute_co
 	}
 }
 
-static float quadricError(const Quadric& Q, const Vector3& v)
+static float quadricEval(const Quadric& Q, const Vector3& v)
 {
 	float rx = Q.b0;
 	float ry = Q.b1;
@@ -621,6 +621,12 @@ static float quadricError(const Quadric& Q, const Vector3& v)
 	r += ry * v.y;
 	r += rz * v.z;
 
+	return r;
+}
+
+static float quadricError(const Quadric& Q, const Vector3& v)
+{
+	float r = quadricEval(Q, v);
 	float s = Q.w == 0.f ? 0.f : 1.f / Q.w;
 
 	return fabsf(r) * s;
@@ -628,26 +634,7 @@ static float quadricError(const Quadric& Q, const Vector3& v)
 
 static float quadricError(const Quadric& Q, const QuadricGrad* G, size_t attribute_count, const Vector3& v, const float* va)
 {
-	float rx = Q.b0;
-	float ry = Q.b1;
-	float rz = Q.b2;
-
-	rx += Q.a10 * v.y;
-	ry += Q.a21 * v.z;
-	rz += Q.a20 * v.x;
-
-	rx *= 2;
-	ry *= 2;
-	rz *= 2;
-
-	rx += Q.a00 * v.x;
-	ry += Q.a11 * v.y;
-	rz += Q.a22 * v.z;
-
-	float r = Q.c;
-	r += rx * v.x;
-	r += ry * v.y;
-	r += rz * v.z;
+	float r = quadricEval(Q, v);
 
 	// see quadricFromAttributes for general derivation; here we need to add the parts of (eval(pos) - attr)^2 that depend on attr
 	for (size_t k = 0; k < attribute_count; ++k)