gltfpack: Combine variant quantization bounds

When a mesh uses multiple materials through the use of variants, we need
to make sure that the quantization parameters for UV for a given
material incorporate all meshes that refer to it, including variants.

In addition, when processing a mesh, we need to take all variant
requirements into account, so that e.g. if only one material needs a
second UV set, we still include it even if it's a variant.

This isn't quite correct in cases when materials are used in other
meshes as well, since we need to ensure that the quantization data is
the same across all materials.

gltf/gltfpack.cpp

@@ -342,7 +342,15 @@ static void process(cgltf_data* data, const char* input_path, const char* output
 	{
 		Mesh& mesh = meshes[i];
 		MaterialInfo mi = mesh.material ? materials[mesh.material - data->materials] : MaterialInfo();
-		// TODO: variants
+
+		// merge material requirements across all variants
+		for (size_t j = 0; j < mesh.variants.size(); ++j)
+		{
+			MaterialInfo vi = materials[mesh.variants[j].material - data->materials];
+
+			mi.needsTangents |= vi.needsTangents;
+			mi.textureSetMask |= vi.textureSetMask;
+		}
 
 		processMesh(mesh, mi, settings);
 	}

gltf/stream.cpp

@@ -24,6 +24,15 @@ struct Bounds
 	{
 		return min.f[0] <= max.f[0] && min.f[1] <= max.f[1] && min.f[2] <= max.f[2] && min.f[3] <= max.f[3];
 	}
+
+	void merge(const Bounds& other)
+	{
+		for (int k = 0; k < 4; ++k)
+		{
+			min.f[k] = min.f[k] < other.min.f[k] ? min.f[k] : other.min.f[k];
+			max.f[k] = max.f[k] > other.max.f[k] ? max.f[k] : other.max.f[k];
+		}
+	}
 };
 
 static void updateAttributeBounds(const Mesh& mesh, cgltf_attribute_type type, Bounds& b)
@@ -107,14 +116,14 @@ void prepareQuantizationTexture(cgltf_data* data, std::vector<QuantizationTextur
 	{
 		const Mesh& mesh = meshes[i];
 
-		if (!mesh.material)
-			continue;
+		Bounds uvb;
+		updateAttributeBounds(mesh, cgltf_attribute_type_texcoord, uvb);
 
-		size_t mi = mesh.material - data->materials;
-		assert(mi < bounds.size());
+		if (mesh.material)
+			bounds[mesh.material - data->materials].merge(uvb);
 
-		updateAttributeBounds(mesh, cgltf_attribute_type_texcoord, bounds[mi]);
-		// TODO: variants
+		for (size_t j = 0; j < mesh.variants.size(); ++j)
+			bounds[mesh.variants[j].material - data->materials].merge(uvb);
 	}
 
 	for (size_t i = 0; i < result.size(); ++i)