Skip initial buffer swap when single-buffered

This skips the buffer swap after the initial glClear performed during
window creation, if the window is single-buffered.  This call confused
apitrace into thinking the window was double-buffered.

Fixes #1873.

README.md

@@ -140,6 +140,7 @@ information on what to include when reporting a bug.
  - Bugfix: Built-in mappings failed because some OEMs re-used VID/PID (#1583)
  - Bugfix: Some extension loader headers did not prevent default OpenGL header
    inclusion (#1695)
+ - Bugfix: Buffers were swapped at creation on single-buffered windows (#1873)
  - [Win32] Added the `GLFW_WIN32_KEYBOARD_MENU` window hint for enabling access
            to the window menu
  - [Win32] Added a version info resource to the GLFW DLL

src/context.c

@@ -570,6 +570,8 @@ GLFWbool _glfwRefreshContextAttribs(_GLFWwindow* window,
         PFNGLCLEARPROC glClear = (PFNGLCLEARPROC)
             window->context.getProcAddress("glClear");
         glClear(GL_COLOR_BUFFER_BIT);
+
+        if (window->doublebuffer)
             window->context.swapBuffers(window);
     }