[imgui] Fixes to work with new api.

- Font atlas is now owned by context.
- Switch from deprecated io.RenderDrawListsFn callback to dedicated call in window.cc.
- Replaced deprecated ImGuiCol_ModalWindowDarkening with ImGuiCol_ModalWindowDimBg.
- Replaced deprecated SetScrollHere() with SetScrollHereY().
- Replaced deprecated GetContentRegionAvailWidth() with GetContentRegionAvail().x.
- Replaced deprecated ShowTestWindow() with ShowDemoWindow().
- Replaced deprecated ImGuiCol_ChildWindowBg with ImGuiCol_ChildBg.
- Replaced deprecated SetNextTreeNodeOpen() with SetNextItemOpen().

src/xenia/ui/window_demo.cc

@@ -102,7 +102,7 @@ int window_demo_main(const std::vector<std::wstring>& args) {
   window->on_painting.AddListener([&](xe::ui::UIEvent* e) {
     auto& io = window->imgui_drawer()->GetIO();
 
-    ImGui::ShowTestWindow();
+    ImGui::ShowDemoWindow();
     ImGui::ShowMetricsWindow();
 
     // Continuous paint.