obj2sdf, add option to merge shapes with same materials, add support for transparent objects

tiny_obj_loader, load the "d" and "Tr" transparant tag

examples/ThirdPartyLibs/Wavefront/tiny_obj_loader.cpp

@@ -328,6 +328,7 @@ void InitMaterial(material_t& material) {
     material.emission[i] = 0.f;
   }
   material.shininess = 1.f;
+  material.transparency = 1.f;
 }
 
 std::string LoadMtl (
@@ -465,6 +466,20 @@ std::string LoadMtl (
       continue;
     }
 
+	// transparency
+	if (token[0] == 'T' && token[1] == 'r' && isSpace(token[2])) {
+		token += 2;
+		material.transparency = parseFloat(token);
+		continue;
+	}
+
+	// transparency
+	if (token[0] == 'd' && isSpace(token[1])) {
+		token += 1;
+		material.transparency = parseFloat(token);
+		continue;
+	}
+
     // ambient texture
     if ((0 == strncmp(token, "map_Ka", 6)) && isSpace(token[6])) {
       token += 7;

examples/ThirdPartyLibs/Wavefront/tiny_obj_loader.h

@@ -22,6 +22,7 @@ typedef struct
     float transmittance[3];
     float emission[3];
     float shininess;
+	float transparency;
 
     std::string ambient_texname;
     std::string diffuse_texname;