Portability fixes to OpenGL example application (courtesy of djoshea)

examples/opengl_example/main.cpp

@@ -191,13 +191,13 @@ void InitGL()
     if (!glfwInit())
         exit(1);
 
-	//glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);
-	//glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 0);
-	//glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);
-	//glfwWindowHint(GLFW_OPENGL_FORWARD_COMPAT, GL_TRUE);
+	glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);
+	glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 2);
+	glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);
+	glfwWindowHint(GLFW_OPENGL_FORWARD_COMPAT, GL_TRUE);
 	glfwWindowHint(GLFW_REFRESH_RATE, 60);
 	glfwWindowHint(GLFW_RESIZABLE, GL_FALSE);
-	window = glfwCreateWindow(1280, 720, "ImGui OpenGL example", nullptr, nullptr);
+	window = glfwCreateWindow(1280, 720, "ImGui OpenGL example", NULL, NULL);
 	glfwMakeContextCurrent(window);
 
 	glfwSetKeyCallback(window, glfw_key_callback);
@@ -207,8 +207,10 @@ void InitGL()
 	glewExperimental = GL_TRUE;
 	glewInit();
 
-	GLenum err = GL_NO_ERROR;
-	err = glGetError(); IM_ASSERT(err == GL_NO_ERROR);
+	// After calling glewInit() our GL error state may be GL_INVALID_ENUM
+	const GLenum err = glGetError();
+	(void)err;
+    IM_ASSERT(err == GL_NO_ERROR || err == GL_INVALID_ENUM);
 }
 
 void InitImGui()