Added a LoadObj function that reads from a std::istream.

+ Added a LoadObj function that reads from a std::istream.  This should allow
   more generic usage and possibly make testing a little easier.
   o This LoadObj accepts a function that returns a
     std::unique_ptr<std::istream> for a material.
 + Modified LoadMtl to read from a std::istream to allow more generic
   usage.
 + Modified test.cc to check that the changes work as expected and nothing was
   broken.

Tests:
 + Compiled test.cc, checked diff of output against pre change output.
   Same output where expected.

test.cc

@@ -3,23 +3,15 @@
 #include <cstdio>
 #include <cstdlib>
 #include <cassert>
+#include <cstddef>
 #include <iostream>
+#include <sstream>
+#include <fstream>
+#include <functional>
+#include <memory>
 
-static bool
-TestLoadObj(
-  const char* filename,
-  const char* basepath = NULL)
+static void PrintInfo(const std::vector<tinyobj::shape_t>& shapes)
 {
-  std::cout << "Loading " << filename << std::endl;
-
-  std::vector<tinyobj::shape_t> shapes;
-  std::string err = tinyobj::LoadObj(shapes, filename, basepath);
-
-  if (!err.empty()) {
-    std::cerr << err << std::endl;
-    return false;
-  }
-
   std::cout << "# of shapes : " << shapes.size() << std::endl;
 
   for (size_t i = 0; i < shapes.size(); i++) {
@@ -53,14 +45,124 @@ TestLoadObj(
     printf("  material.map_Kd = %s\n", shapes[i].material.diffuse_texname.c_str());
     printf("  material.map_Ks = %s\n", shapes[i].material.specular_texname.c_str());
     printf("  material.map_Ns = %s\n", shapes[i].material.normal_texname.c_str());
-    std::map<std::string, std::string>::iterator it(shapes[i].material.unknown_parameter.begin());
-    std::map<std::string, std::string>::iterator itEnd(shapes[i].material.unknown_parameter.end());
+    auto it = shapes[i].material.unknown_parameter.begin();
+    auto itEnd = shapes[i].material.unknown_parameter.end();
     for (; it != itEnd; it++) {
       printf("  material.%s = %s\n", it->first.c_str(), it->second.c_str());
     }
     printf("\n");
   }
+}
 
+static bool
+TestLoadObj(
+  const char* filename,
+  const char* basepath = NULL)
+{
+  std::cout << "Loading " << filename << std::endl;
+
+  std::vector<tinyobj::shape_t> shapes;
+  std::string err = tinyobj::LoadObj(shapes, filename, basepath);
+
+  if (!err.empty()) {
+    std::cerr << err << std::endl;
+    return false;
+  }
+
+  PrintInfo(shapes);
+
+  return true;
+}
+
+
+static bool
+TestStreamLoadObj()
+{
+  std::cout << "Stream Loading " << std::endl;
+
+  std::stringstream objStream;
+  objStream 
+    << "mtllib cube.mtl\n"
+    "\n"
+    "v 0.000000 2.000000 2.000000\n"
+    "v 0.000000 0.000000 2.000000\n"
+    "v 2.000000 0.000000 2.000000\n"
+    "v 2.000000 2.000000 2.000000\n"
+    "v 0.000000 2.000000 0.000000\n"
+    "v 0.000000 0.000000 0.000000\n"
+    "v 2.000000 0.000000 0.000000\n"
+    "v 2.000000 2.000000 0.000000\n"
+    "# 8 vertices\n"
+    "\n"
+    "g front cube\n"
+    "usemtl white\n"
+    "f 1 2 3 4\n"
+    "g back cube\n"
+    "# expects white material\n"
+    "f 8 7 6 5\n"
+    "g right cube\n"
+    "usemtl red\n"
+    "f 4 3 7 8\n"
+    "g top cube\n"
+    "usemtl white\n"
+    "f 5 1 4 8\n"
+    "g left cube\n"
+    "usemtl green\n"
+    "f 5 6 2 1\n"
+    "g bottom cube\n"
+    "usemtl white\n"
+    "f 2 6 7 3\n"
+    "# 6 elements";
+   
+  auto getMatFileIStreamFunc = 
+    [](const std::string& matId)
+    {           
+      if (matId == "cube.mtl") {
+      
+        std::unique_ptr<std::stringstream> matStream( 
+          new std::stringstream(
+            "newmtl white\n"
+            "Ka 0 0 0\n"
+            "Kd 1 1 1\n"
+            "Ks 0 0 0\n"
+            "\n"
+            "newmtl red\n"
+            "Ka 0 0 0\n"
+            "Kd 1 0 0\n"
+            "Ks 0 0 0\n"
+            "\n"
+            "newmtl green\n"
+            "Ka 0 0 0\n"
+            "Kd 0 1 0\n"
+            "Ks 0 0 0\n"
+            "\n"
+            "newmtl blue\n"
+            "Ka 0 0 0\n"
+            "Kd 0 0 1\n"
+            "Ks 0 0 0\n"
+            "\n"
+            "newmtl light\n"
+            "Ka 20 20 20\n"
+            "Kd 1 1 1\n"
+            "Ks 0 0 0"));
+
+        return matStream;
+      }
+
+      std::unique_ptr<std::stringstream> emptyUP( nullptr );
+      return emptyUP;
+    };
+    
+  std::vector<tinyobj::shape_t> shapes;
+  std::string err = tinyobj::LoadObj(shapes, objStream, getMatFileIStreamFunc);    
+  
+  if (!err.empty()) {
+    std::cerr << err << std::endl;
+    return false;
+  }
+
+  PrintInfo(shapes);
+    
   return true;
 }
 
@@ -79,7 +181,8 @@ main(
   } else {
     assert(true == TestLoadObj("cornell_box.obj"));
     assert(true == TestLoadObj("cube.obj"));
+    assert(true == TestStreamLoadObj());
   }
-
+  
   return 0;
 }