gltfpack: Warn if input glTF file contains experimental versions

... or legacy versions (e.g. version 0 for index codec). glTF
specification of EXT_meshopt_compression explicitly only supports
vertex 0 / index 1.

Author: zeux

gltf/parsegltf.cpp

@@ -518,6 +518,8 @@ static bool freeUnusedBuffers(cgltf_data* data)
 
 static cgltf_result decompressMeshopt(cgltf_data* data)
 {
+	bool warned = false;
+
 	for (size_t i = 0; i < data->buffer_views_count; ++i)
 	{
 		if (!data->buffer_views[i].has_meshopt_compression)
@@ -536,18 +538,22 @@ static cgltf_result decompressMeshopt(cgltf_data* data)
 		data->buffer_views[i].data = result;
 
 		int rc = -1;
+		bool warn = false;
 
 		switch (mc->mode)
 		{
 		case cgltf_meshopt_compression_mode_attributes:
+			warn = meshopt_decodeVertexVersion(source, mc->size) != 0;
 			rc = meshopt_decodeVertexBuffer(result, mc->count, mc->stride, source, mc->size);
 			break;
 
 		case cgltf_meshopt_compression_mode_triangles:
+			warn = meshopt_decodeIndexVersion(source, mc->size) != 1;
 			rc = meshopt_decodeIndexBuffer(result, mc->count, mc->stride, source, mc->size);
 			break;
 
 		case cgltf_meshopt_compression_mode_indices:
+			warn = meshopt_decodeIndexVersion(source, mc->size) != 1;
 			rc = meshopt_decodeIndexSequence(result, mc->count, mc->stride, source, mc->size);
 			break;
 
@@ -558,6 +564,12 @@ static cgltf_result decompressMeshopt(cgltf_data* data)
 		if (rc != 0)
 			return cgltf_result_io_error;
 
+		if (warn && !warned)
+		{
+			fprintf(stderr, "Warning: EXT_meshopt_compression data uses versions outside of the glTF specification (vertex 0 / index 1 expected)\n");
+			warned = true;
+		}
+
 		switch (mc->filter)
 		{
 		case cgltf_meshopt_compression_filter_octahedral: