Fix a bug in CoordinateDescentMinimizer

CoordinateDescentMinimizer optimizes one parameter block at a time.
To do this, it manipulates the parameter block object. It was doing
so inconsistently, where the tangent space offset was being set to
zero but the ambient state offset was not being set to zero. This
did not cause problems because these offsets were not really being
used inside the CoordinateDescentMinimizer. However the recent
change which parallelizes Program::Plus uncovered this bug.

The reason this bug was not caught was because, CoordinateDescentMinimizer
does not have any tests. I will fix this shortly, but in the interim
to unbreak inner iterations at head, this small change should go in.

Change-Id: I55d2698e8509f9cb5751e7a5180427129d86e720

internal/ceres/coordinate_descent_minimizer.cc

diff --git a/internal/ceres/coordinate_descent_minimizer.cc b/internal/ceres/coordinate_descent_minimizer.cc
index 8ecdeff..de22822 100644
--- a/internal/ceres/coordinate_descent_minimizer.cc
+++ b/internal/ceres/coordinate_descent_minimizer.cc
@@ -167,9 +167,11 @@
           ParameterBlock* parameter_block = parameter_blocks_[j];
           const int old_index = parameter_block->index();
           const int old_delta_offset = parameter_block->delta_offset();
+          const int old_state_offset = parameter_block->state_offset();
           parameter_block->SetVarying();
           parameter_block->set_index(0);
           parameter_block->set_delta_offset(0);
+          parameter_block->set_state_offset(0);
 
           Program inner_program;
           inner_program.mutable_parameter_blocks()->push_back(parameter_block);
@@ -186,11 +188,12 @@
           Solver::Summary inner_summary;
           Solve(&inner_program,
                 linear_solvers[thread_id].get(),
-                parameters + parameter_block->state_offset(),
+                parameters + old_state_offset,
                 &inner_summary);
 
           parameter_block->set_index(old_index);
           parameter_block->set_delta_offset(old_delta_offset);
+          parameter_block->set_state_offset(old_state_offset);
           parameter_block->SetState(parameters +
                                     parameter_block->state_offset());
           parameter_block->SetConstant();