Get rid of redundant function evaluations in LineSearchMinimizer

1. Replace LineSearch::Summary::optimal_step_size with
   LineSearch:Summary::optimal_point which is a FunctionSample.
2. Add the actual vector position and vector gradient of the
   point in the FunctionSample
3. Use the above two to get rid of an extraneous function evalation
   in LineSearchMinimizer.

Runtime performance is almost 2x improved as a result.

Thanks to @svenpilz for reporting this.

https://github.com/ceres-solver/ceres-solver/issues/296

Change-Id: Iebf2db7acecb2c95c9b1683b73cdc5faab78b02e

internal/ceres/function_sample.cc

@@ -34,6 +34,35 @@
 namespace ceres {
 namespace internal {
 
+FunctionSample::FunctionSample()
+    : x(0.0),
+      vector_x_is_valid(false),
+      value(0.0),
+      value_is_valid(false),
+      vector_gradient_is_valid(false),
+      gradient(0.0),
+      gradient_is_valid(false) {}
+
+FunctionSample::FunctionSample(const double x, const double value)
+    : x(x),
+      vector_x_is_valid(false),
+      value(value),
+      value_is_valid(true),
+      vector_gradient_is_valid(false),
+      gradient(0.0),
+      gradient_is_valid(false) {}
+
+FunctionSample::FunctionSample(const double x,
+                               const double value,
+                               const double gradient)
+    : x(x),
+      vector_x_is_valid(false),
+      value(value),
+      value_is_valid(true),
+      vector_gradient_is_valid(false),
+      gradient(gradient),
+      gradient_is_valid(true) {}
+
 std::string FunctionSample::ToDebugString() const {
   return StringPrintf("[x: %.8e, value: %.8e, gradient: %.8e, "
                       "value_is_valid: %d, gradient_is_valid: %d]",