Use geom margin in the midphase.

At the leaves of the midphase trees in `mj_collideTree`, a bounding sphere test is performed. The margin was incorrectly set to zero instead of reading it from `geom_margin`.

PiperOrigin-RevId: 559782262
Change-Id: Icfceb8445bbf6977582e0061566d7bd4def2f0ae

doc/changelog.rst

@@ -43,6 +43,11 @@ Python bindings
 7. Fixed `#870 <https://github.com/deepmind/mujoco/issues/870>`__ where calling ``update_scene`` with an invalid
    camera name used the default camera.
 
+Bug fixes
+^^^^^^^^^
+
+#. Fixed a bug that was causing the geom margins to be ignored during the midphase.
+
 
 Version 2.3.7 (July 20, 2023)
 -----------------------------

src/engine/engine_collision_driver.c

@@ -244,7 +244,8 @@ void mj_collideTree(const mjModel* m, mjData* d, int b1, int b2,
 
     // both are leaves
     if (isleaf1 && isleaf2 && nodeid1 != -1 && nodeid2 != -1) {
-      if (mj_collideSphere(m, d, nodeid1, nodeid2, /*margin=*/ 0)) {
+      if (mj_collideSphere(m, d, nodeid1, nodeid2, m->geom_margin[nodeid1] +
+                                                   m->geom_margin[nodeid2])) {
         if (mj_collideOBB(m->geom_aabb + 6*nodeid1, m->geom_aabb + 6*nodeid2,
                           d->geom_xpos + 3*nodeid1, d->geom_xmat + 9*nodeid1,
                           d->geom_xpos + 3*nodeid2, d->geom_xmat + 9*nodeid2,