Avoid duplicating path string in ExpandFilePath

[moretromain]

Changing signature of ExpandFilePath() to avoid extra string copies/allocations

[syoyo]

Changing API will affect programs using tinygltf, so I don't want to merge this change unless this change give significant performance improvement(but looks no)

[moretromain]

I understand this affects the API and might break some existing code. On our end, we use a custom set of FSCallbacks, and our ExpandFilePath() doesn't do anything. But with the actual signature, we still need to return an extra copy of the input argument.
We're in a context where we really need to minimize allocations and string copies as much as we can, so I thought this would be a welcome change. I understand if you don't want to break the current API though..
Thanks!