MathUtils: declare fastATan2's Pi where its lambdas can use it

The local constexpr T Pi sits outside the two lambdas that read it. For a builtin
T every use is an lvalue-to-rvalue conversion on a constant, so Pi is not
odr-used and needs no capture. For a class type the operator call odr-uses it,
which a lambda with no capture-default cannot do.

Capturing it would not help either, because C, Pi025 and Pi075 are constexpr and
a captured variable is not a constant expression. Declaring Pi inside each lambda
satisfies both, and costs nothing: it is folded at compile time either way.

Author: ktf

Common/MathUtils/include/MathUtils/detail/trigonometric.h

@@ -275,12 +275,11 @@ GPUhdi() constexpr T fastATan2(T y, T x)
   // Average inaccuracy: 0.00048
   // Max inaccuracy: 0.00084
   // Speed: 6.2 times faster than atan2f()
-  constexpr T Pi = 3.1415926535897932384626433832795;
-
   auto atan = [](T a) -> T {
     // returns the arctan for the angular range [-Pi/4, Pi/4]
     // the polynomial coefficients are taken from:
     // https://stackoverflow.com/questions/42537957/fast-accurate-atan-arctan-approximation-algorithm
+    constexpr T Pi = 3.1415926535897932384626433832795;
     constexpr T A = 0.0776509570923569;
     constexpr T B = -0.287434475393028;
     constexpr T C = ((Pi / 4) - A - B);
@@ -290,6 +289,7 @@ GPUhdi() constexpr T fastATan2(T y, T x)
 
   auto atan2P = [atan](T yy, T xx) -> T {
     // fast atan2(yy,xx) for the angular range [0,+Pi]
+    constexpr T Pi = 3.1415926535897932384626433832795;
     constexpr T Pi025 = 1 * Pi / 4;
     constexpr T Pi075 = 3 * Pi / 4;
     const T x1 = xx + yy; //  point p1 (x1,y1) = (xx,yy) - Pi/4