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/VideoCommon/RenderBase.cpp

@@ -94,6 +94,7 @@ Renderer::Renderer(int backbuffer_width, int backbuffer_height)
   if (SConfig::GetInstance().bWii)
     m_aspect_wide = Config::Get(Config::SYSCONF_WIDESCREEN);
 
+  m_surface_handle = Host_GetRenderHandle();
   m_last_host_config_bits = ShaderHostConfig::GetCurrent().bits;
 }
 
@@ -400,6 +401,11 @@ float Renderer::CalculateDrawAspectRatio() const
   }
 }
 
+bool Renderer::IsHeadless() const
+{
+  return !m_surface_handle;
+}
+
 std::tuple<float, float> Renderer::ScaleToDisplayAspectRatio(const int width,
                                                              const int height) const
 {
@@ -711,7 +717,7 @@ void Renderer::DoDumpFrame()
 void Renderer::UpdateFrameDumpTexture()
 {
   int target_width, target_height;
-  if (!g_ActiveConfig.bInternalResolutionFrameDumps)
+  if (!g_ActiveConfig.bInternalResolutionFrameDumps && !IsHeadless())
   {
     auto target_rect = GetTargetRectangle();
     target_width = target_rect.GetWidth();