Add missing return in Backward distortion functions for near-zero radius guard

Summary:
All 6 `Backward` methods in camera distortion structs (Disto2, Disto24, Disto2468, DistoBrown, Disto62, DistoDualFisheye) have an early-exit guard for near-zero distorted radius: when `rd < epsilon`, the code sets `undistorted = point` (identity undistortion). However, the guard was missing a `return` statement, so execution always fell through to the Newton-Raphson iterative solver.

This makes the guard completely ineffective. For points very near the optical center, the Newton-Raphson solver divides near-zero values by near-zero distortion factors, potentially producing NaN or wildly incorrect undistorted coordinates.

This diff adds `return;` after the identity assignment in all 6 Backward methods, making the early-exit guard take effect.

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overriding_review_checks_triggers_an_audit_and_retroactive_review
Oncall Short Name: mrservice_triage

Differential Revision: D100590112

fbshipit-source-id: 40839c830f2424126e1727906324f635d5c5423a

Author: Fabian Schenk

opensfm/src/geometry/camera_distortions_functions.h

@@ -54,6 +54,7 @@ struct Disto2 : Functor<2, 1, 2> {
     if (rd < T(std::numeric_limits<double>::epsilon())) {
       undistorted[0] = point[0];
       undistorted[1] = point[1];
+      return;
     }
 
     // Compute undistorted radius
@@ -152,6 +153,7 @@ struct Disto24 : Functor<2, 2, 2> {
     if (rd < T(std::numeric_limits<double>::epsilon())) {
       undistorted[0] = point[0];
       undistorted[1] = point[1];
+      return;
     }
 
     // Compute undistorted radius
@@ -268,6 +270,7 @@ struct Disto2468 : Functor<2, 4, 2> {
     if (rd < T(std::numeric_limits<double>::epsilon())) {
       undistorted[0] = point[0];
       undistorted[1] = point[1];
+      return;
     }
 
     // Compute undistorted radius
@@ -421,6 +424,7 @@ struct Disto62 : Functor<2, 8, 2> {
     if (rd < T(std::numeric_limits<double>::epsilon())) {
       undistorted[0] = point[0];
       undistorted[1] = point[1];
+      return;
     }
     Eigen::Map<const Vec2<T>> mapped_point(point);
     // Compute undistorted radius
@@ -627,6 +631,7 @@ struct Disto624 : Functor<2, 12, 2> {
     if (rd < T(std::numeric_limits<double>::epsilon())) {
       undistorted[0] = point[0];
       undistorted[1] = point[1];
+      return;
     }
     Eigen::Map<const Vec2<T>> mapped_point(point);
     // Compute undistorted radius
@@ -774,6 +779,7 @@ struct DistoBrown : Functor<2, 5, 2> {
     if (rd < T(std::numeric_limits<double>::epsilon())) {
       undistorted[0] = point[0];
       undistorted[1] = point[1];
+      return;
     }
 
     Eigen::Map<const Vec2<T>> mapped_point(point);