Math/Angle: don't use optimised multiplication for Angle*fixed

Reverts the second part of e4b9b576.  Turns out that the
CatmullRomInterpolator does care a lot about the bits discarded by the
fast_mult() call, because it passes very small values (TRAC #2537).
The unfortunate bottom line is we can't predict the dynamics of the
two factors, and we must not discard any bit in the default operator
without knowing from where this was called.
RoutePolar: Optimized Initialise() method

The IndexToAngle() function performed a division, a multiplication and
a substraction for every call in the for loop of the Initialise() method.
This could be reduced to one constexpr constant and one substraction for
each iteration.