Code review comment for lp:~cemil-azizoglu/mir/add-create-module-context-fn

*Needs Discussion*

I'm just pondering an alternative approach: adding a method to mg::Platform. Vis:

  auto create_input_platform(...) -> UniqueModulePtr<input::Platform>

It the user has specified an input platform this can be ignored, otherwise call this and if it returns non-nullptr then use the result. Otherwise Mir looks for a suitable module elsewhere as currently.