[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.cc

@@ -210,6 +210,7 @@ void Window::OnPaint(UIEvent* e) {
 
   // Flush ImGui buffers before we swap.
   ImGui::Render();
+  imgui_drawer_->RenderDrawLists();
 
   ForEachListener([e](auto listener) { listener->OnPainted(e); });
   on_painted(e);