Dear ImGui outputs 16-bit vertex indices by default.
Allegro doesn't support them natively, so we have two solutions: convert the indices manually in imgui_impl_allegro5.cpp, or compile dear imgui with 32-bit indices.
You can either modify imconfig.h that comes with Dear ImGui (easier), or set a C++ preprocessor option IMGUI_USER_CONFIG to find to a filename.
We are providing `imconfig_allegro5.h` that enables 32-bit indices.
Note that the backend supports _BOTH_ 16-bit and 32-bit indices, but 32-bit indices will be slightly faster as they won't require a manual conversion.