Make GLFW_DOUBLEBUFFER a window attribute

src/internal.h

@@ -387,6 +387,7 @@ struct _GLFWwindow
     GLFWbool            mousePassthrough;
     GLFWbool            shouldClose;
     void*               userPointer;
+    GLFWbool            doublebuffer;
     GLFWvidmode         videoMode;
     _GLFWmonitor*       monitor;
     _GLFWcursor*        cursor;