lib/math/rational.c: fix possible incorrect result from rational fractions helper
In some cases the previous algorithm would not return the closest approximation. This would happen when a semi-convergent was the closest, as the previous algorithm would only consider convergents. As an example, consider an initial value of 5/4, and trying to find the closest approximation with a maximum of 4 for numerator and denominator. The previous algorithm would return 1/1 as the closest approximation, while this version will return the correct answer of 4/3. To do this, the main loop performs effectively the same operations as it did before. It must now keep track of the last three approximations, n2/d2 .. n0/d0, while before it only needed the last two. If an exact answer is not found, the algorithm will now calculate the best semi-convergent term, t, which is a single expression with two divisions: min((max_numerator - n0) / n1, (max_denominator - d0) / d1) This will be used if it is better than previou...
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