Add backend-specific functions for checking if a file is in its format

[Clownacy]

Right now clownaudio just uses the backend's 'Create' function to see if the loaded file is in that backend's format, but that's a whole lot of overhead.

Certain libraries (I think libVorbis is one of them) have specific functions for testing if the provided buffer contains valid data. Using those would be nice.

Backends whose libraries don't provide such a feature can still rely on the old 'just try opening the file' trick.