A hacky fix for the Eigen::FullPivLU changes.

Changes in Eigen's implementation for FullPivLU significantly
degraded the performance of the line search as reported by Weiguang.

We do not completely understand what is going on, as Eigen's changes
seem sane. So for now, this change explicitly works around the
changes made by Eigen to restore the performance of the line search.

Figuring out the underlying problem and fixing it remains an open
issue.

https://github.com/ceres-solver/ceres-solver/issues/248

Change-Id: I9993d73a09dc990ab567ce6bc447f16eac74abec

internal/ceres/polynomial.cc

diff --git a/internal/ceres/polynomial.cc b/internal/ceres/polynomial.cc
index 13bf8ed..aef17bb 100644
--- a/internal/ceres/polynomial.cc
+++ b/internal/ceres/polynomial.cc
@@ -370,7 +370,10 @@
     }
   }
 
-  return lhs.fullPivLu().solve(rhs);
+  // TODO(sameeragarwal): This is a hack.
+  // https://github.com/ceres-solver/ceres-solver/issues/248
+  Eigen::FullPivLU<Matrix> lu(lhs);
+  return lu.setThreshold(0.0).solve(rhs);
 }
 
 void MinimizeInterpolatingPolynomial(const vector<FunctionSample>& samples,