Fix handling missing skins using ufbx importer

Previously, _asset_parse_skins() would mess with the order of skin indices.
However, the rest of the code expected these to match to ufbx skin indices.
To fix this, retain the original skin indices in FBXState::original_skin_indices.

modules/fbx/fbx_document.cpp

@@ -693,10 +693,7 @@ Error FBXDocument::_parse_meshes(Ref<FBXState> p_state) {
 
 				// Find the first imported skin deformer
 				for (ufbx_skin_deformer *fbx_skin : fbx_mesh->skin_deformers) {
-					if (!p_state->skin_indices.has(fbx_skin->typed_id)) {
-						continue;
-					}
-					GLTFSkinIndex skin_i = p_state->skin_indices[fbx_skin->typed_id];
+					GLTFSkinIndex skin_i = p_state->original_skin_indices[fbx_skin->typed_id];
 					if (skin_i < 0) {
 						continue;
 					}
@@ -2341,7 +2338,7 @@ Error FBXDocument::_parse_skins(Ref<FBXState> p_state) {
 	HashMap<GLTFNodeIndex, bool> joint_mapping;
 
 	for (const ufbx_skin_deformer *fbx_skin : fbx_scene->skin_deformers) {
-		if (fbx_skin->clusters.count == 0) {
+		if (fbx_skin->clusters.count == 0 || fbx_skin->weights.count == 0) {
 			p_state->skin_indices.push_back(-1);
 			continue;
 		}
@@ -2387,8 +2384,9 @@ Error FBXDocument::_parse_skins(Ref<FBXState> p_state) {
 			}
 		}
 	}
+	p_state->original_skin_indices = p_state->skin_indices.duplicate();
 	Error err = SkinTool::_asset_parse_skins(
-			p_state->skin_indices.duplicate(),
+			p_state->original_skin_indices,
 			p_state->skins.duplicate(),
 			p_state->nodes.duplicate(),
 			p_state->skin_indices,

modules/fbx/fbx_state.h

@@ -53,6 +53,7 @@ class FBXState : public GLTFState {
 	HashMap<Pair<uint64_t, uint64_t>, GLTFTextureIndex, PairHash<uint64_t, uint64_t>> albedo_transparency_textures;
 
 	Vector<GLTFSkinIndex> skin_indices;
+	Vector<GLTFSkinIndex> original_skin_indices;
 	HashMap<ObjectID, GLTFSkeletonIndex> skeleton3d_to_fbx_skeleton;
 	HashMap<ObjectID, HashMap<ObjectID, GLTFSkinIndex>> skin_and_skeleton3d_to_fbx_skin;
 	HashSet<String> unique_mesh_names; // Not in GLTFState because GLTFState prefixes mesh names with the scene name (or _)