Demonstrate standard and docking ImGui variants

[antono3]

Summary

• display the selected upstream ImGui variant in the Vulkan demo
• create a main dockspace and expose optional platform viewports on docking builds
• retain ordinary floating-window behavior on standard builds through the same source

Verification

• compiled and visually exercised with Dear ImGui 1.92.9b standard bindings
• compiled and visually exercised with Dear ImGui 1.92.9b docking bindings
• confirmed docking into a tabbed layout
• confirmed the platform-viewports toggle enables without errors

Depends on antono2/imgui#10 and antono2/imgui#11.