Update bisect to return a single root with closely spaced roots.

This is the current practice in the non-sturm chain based solvers and
expected by at least some primitive objects. Root returned is the middle
of the tiny range at which we stop bisection.

Author: wfpokorny

source/core/math/polynomialsolver.cpp

@@ -563,7 +563,8 @@ static int sbisect(int np, const polynomial *sseq, DBL min_value, DBL max_value,
         {
             if ((max_value - min_value) < SBISECT_MULT_ROOT_THRESHOLD)
             {
-                return(0);
+                roots[0] = mid;
+                return(1);
             }
 
             n1 = sbisect(np, sseq, min_value, mid, atmin, atmid, roots);