Avoid doubly mutating the same Skin in rest fixer

Fixes a regression from #77123 that was caused by removal of ibm_diffs variable.
This replaced idempotent code with code that applied an offset each time.
If the same Skin was visited multiple times, this caused an incorrect result.

(cherry picked from commit 5ae311e577b508629712fe25e34b03c6d942e3cb)

editor/import/post_import_plugin_skeleton_rest_fixer.cpp

@@ -609,8 +609,8 @@ void PostImportPluginSkeletonRestFixer::internal_process(InternalImportCategory
 		if (is_rest_changed) {
 			// Fix skin.
 			{
+				HashSet<Ref<Skin>> mutated_skins;
 				TypedArray<Node> nodes = p_base_scene->find_children("*", "ImporterMeshInstance3D");
-				int skin_idx = 0;
 				while (nodes.size()) {
 					ImporterMeshInstance3D *mi = Object::cast_to<ImporterMeshInstance3D>(nodes.pop_back());
 					ERR_CONTINUE(!mi);
@@ -619,6 +619,10 @@ void PostImportPluginSkeletonRestFixer::internal_process(InternalImportCategory
 					if (skin.is_null()) {
 						continue;
 					}
+					if (mutated_skins.has(skin)) {
+						continue;
+					}
+					mutated_skins.insert(skin);
 
 					Node *node = mi->get_node(mi->get_skeleton_path());
 					ERR_CONTINUE(!node);
@@ -638,8 +642,6 @@ void PostImportPluginSkeletonRestFixer::internal_process(InternalImportCategory
 							skin->set_bind_pose(i, adjust_transform * skin->get_bind_pose(i));
 						}
 					}
-
-					skin_idx++;
 				}
 				nodes = src_skeleton->get_children();
 				while (nodes.size()) {