Fix T64533: Using "X-Axis Mirror" while posing with auto keyframe on does not keyframe the mirrored bone

Use an additional pose bone flag so we can keep track of mirrored bones that should be autokeyframed.

Reviewed By: Brecht

Differential Revision: https://developer.blender.org/D5033

source/blender/editors/transform/transform_generics.c

@@ -819,6 +819,9 @@ static void pose_transform_mirror_update(Object *ob, PoseInitData_Mirror *pid)
           pid++;
         }
         BKE_pchan_apply_mat4(pchan, pchan_mtx_final, false);
+
+        /* set flag to let autokeyframe know to keyframe the mirrred bone */
+        pchan->bone->flag |= BONE_TRANSFORM_MIRROR;
       }
     }
   }