Add stream based material reader implementation

tiny_obj_loader.h

@@ -179,6 +179,19 @@ class MaterialFileReader : public MaterialReader {
   std::string m_mtlBasePath;
 };
 
+class MaterialStreamReader : public MaterialReader {
+ public:
+  explicit MaterialStreamReader(std::istream &inStream)
+      : m_inStream(inStream) {}
+  virtual ~MaterialStreamReader() {}
+  virtual bool operator()(const std::string &matId,
+                          std::vector<material_t> *materials,
+                          std::map<std::string, int> *matMap, std::string *err);
+
+ private:
+  std::istream &m_inStream;
+};
+
 /// Loads .obj from a file.
 /// 'attrib', 'shapes' and 'materials' will be filled with parsed shape data
 /// 'shapes' will be filled with parsed shape data
@@ -1010,6 +1023,22 @@ bool MaterialFileReader::operator()(const std::string &matId,
   return true;
 }
 
+bool MaterialStreamReader::operator()(const std::string &matId,
+                                      std::vector<material_t> *materials,
+                                      std::map<std::string, int> *matMap,
+                                      std::string *err) {
+  LoadMtl(matMap, materials, &m_inStream);
+  if (!m_inStream) {
+    std::stringstream ss;
+    ss << "WARN: Material stream in error state."
+       << " Created a default material.";
+    if (err) {
+      (*err) += ss.str();
+    }
+  }
+  return true;
+}
+
 bool LoadObj(attrib_t *attrib, std::vector<shape_t> *shapes,
              std::vector<material_t> *materials, std::string *err,
              const char *filename, const char *mtl_basepath,