Add check for points behind camera in euclidan BA cost functor

In cases keyframes are no so good, algebraic two frames construction
could produce result, for which more aggressive Ceres-based BA code
will fall to a solution for which points goes behind the camera,
which is not so nice.

Seems in newer Ceres returning false from cost functor wouldn't
abort solution, but will restrict solver from moving points behind
the camera.

Works fine in own tests, but requires more tests.

extern/libmv/libmv/simple_pipeline/bundle.cc

@@ -83,6 +83,10 @@ struct OpenCVReprojectionError {
     x[1] += R_t[4];
     x[2] += R_t[5];
 
+    // Prevent points from going behind the camera.
+    if (x[2] < T(0))
+      return false;
+
     // Compute normalized coordinates: x /= x[2].
     T xn = x[0] / x[2];
     T yn = x[1] / x[2];