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/window.c

@@ -205,6 +205,8 @@ GLFWAPI GLFWwindow* glfwCreateWindow(int width, int height,
     window->focusOnShow = wndconfig.focusOnShow;
     window->cursorMode  = GLFW_CURSOR_NORMAL;
 
+    window->doublebuffer = fbconfig.doublebuffer;
+
     window->minwidth    = GLFW_DONT_CARE;
     window->minheight   = GLFW_DONT_CARE;
     window->maxwidth    = GLFW_DONT_CARE;