gltfpack: Limit the number of non-target streams to 16

We currently have a limit of 16 on some stream based processing
algorithms, and it's possible with custom attributes to reach a limit of
16, so for now we just drop extra streams.

gltf/parsegltf.cpp

@@ -5,6 +5,8 @@
 #include <stdlib.h>
 #include <string.h>
 
+static const size_t kMaxStreams = 16;
+
 static const char* getError(cgltf_result result, cgltf_data* data)
 {
 	switch (result)
@@ -211,6 +213,12 @@ static void parseMeshesGltf(cgltf_data* data, std::vector<Mesh>& meshes, std::ve
 					continue;
 				}
 
+				if (result.streams.size() == kMaxStreams)
+				{
+					fprintf(stderr, "Warning: ignoring attribute %s in primitive %d of mesh %d (limit %d reached)\n", attr.name, int(pi), int(mi), int(kMaxStreams));
+					continue;
+				}
+
 				result.streams.push_back(Stream());
 				Stream& s = result.streams.back();