Fix initial value selection in acb_poly_find_roots for overlapping radii #2422
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Fixes #2421.
Explanation: the recently introduced initial value selection (taken from MPSolve) chooses a sequence of radii based on the convex hull of the logarithmic magnitudes of the coefficients, and places points on the circles with those radii with a quasi-uniform angular distribution.
For some polynomials, notably with coefficients in geometric progression, it can happen that one gets two or more instances of the same radius (within rounding error). For unfortunate combinations of degrees and coefficients, it could happen that the same angle was generated twice, resulting in a duplicated initial value. We add a small offset to avoid this possibility.