OGL: Fix headless frame dumping

Also skips swapping the window system buffers in headless mode, as there
may not be a surface which can be swapped in the first place. Instead,
we call glFlush() at the end of a frame in this case.

Source/Core/VideoBackends/Software/SWRenderer.cpp

@@ -45,9 +45,11 @@ void SWRenderer::SwapImpl(AbstractTexture* texture, const EFBRectangle& xfb_regi
 {
   OSD::DoCallbacks(OSD::CallbackType::OnFrame);
 
-  DrawDebugText();
-
-  SWOGLWindow::s_instance->ShowImage(texture, xfb_region);
+  if (!IsHeadless())
+  {
+    DrawDebugText();
+    SWOGLWindow::s_instance->ShowImage(texture, xfb_region);
+  }
 
   UpdateActiveConfig();
 }