FR: constexpr std::string_view to_string methods for all public enums, for debug-printing

Filament has many public enums. It's natural to want to print these out for debuggers, printf statements, and heck, even for AI agents.

Today, some enums have to_string methods. And some don't. The ones that don't, end up getting duplicate to_string methods in libraries that call Filament. These duplicates get out of sync with the enum definitions over time. That's bad. We'd be better off to put the to_string function inside filament itself, next to the enum.

There are some code size increase costs, but I hope that compilers should be able to find and strip dead code and constants.

On https://github.com/google/filament/pull/9975#issuecomment-4422286831, which proposes one such to_string method, @romainguy says

> It would make more sense to do this consistently for all public enums

I agree, let's file this ticket to track.