Add tracking of single-/double-buffering

This adds the state part of 04f21abb52
(Make GLFW_DOUBLEBUFFER a window attribute), but without the associated
API change, because this is needed by later bug fixes that will go into
the stable branch.

src/internal.h

@@ -375,6 +375,7 @@ struct _GLFWwindow
     GLFWbool            focusOnShow;
     GLFWbool            shouldClose;
     void*               userPointer;
+    GLFWbool            doublebuffer;
     GLFWvidmode         videoMode;
     _GLFWmonitor*       monitor;
     _GLFWcursor*        cursor;