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.

(cherry picked from commit 184377b493)

src/context.c

@@ -570,7 +570,9 @@ GLFWbool _glfwRefreshContextAttribs(_GLFWwindow* window,
         PFNGLCLEARPROC glClear = (PFNGLCLEARPROC)
             window->context.getProcAddress("glClear");
         glClear(GL_COLOR_BUFFER_BIT);
-        window->context.swapBuffers(window);
+
+        if (window->doublebuffer)
+            window->context.swapBuffers(window);
     }
 
     glfwMakeContextCurrent((GLFWwindow*) previous);