Add a return parameter to ImGui_Impl...._NewFrame()

Why does CreateDeviceObjects return a bool if the NewFrame doesn't ?
Added the return parameter cause CreateDeviceObjects can fail.
It did fail on DX11 with feature level to 9.

ImGui/impls/windows/imgui_impl_dx11.cpp

@@ -556,8 +556,10 @@ void ImGui_ImplDX11_Shutdown()
     if (g_pd3dDeviceContext) { g_pd3dDeviceContext->Release(); g_pd3dDeviceContext = NULL; }
 }
 
-void ImGui_ImplDX11_NewFrame()
+bool ImGui_ImplDX11_NewFrame()
 {
     if (!g_pFontSampler)
-        ImGui_ImplDX11_CreateDeviceObjects();
+        return ImGui_ImplDX11_CreateDeviceObjects();
+
+    return true;
 }