Allow retrieval of Picture's underlying Dav1dPicture

[SuspiciousDuck]

I want to get the pointer to a decoded frame's Dav1dPicture so I can pass it to libplacebo's pl_frame_from_dav1dpicture, but it doesn't look like there's a function which can do that.

[sdroege]

I think adding various as_ptr() methods to the types here would make sense. Would that be sufficient for your use-case? If so, do you want to create a PR?

[SuspiciousDuck]

Yes, it would be sufficient. I'm too lazy to make a PR, but this is the only change I made to dav1d-rs.

impl<A: PictureAllocator> AsRef<Dav1dPicture> for Picture<A> {
    fn as_ref(&self) -> &Dav1dPicture {
        &self.inner.pic
    }
}

[sdroege]

I guess if you don't actually need this we can leave it until someone does and actually makes use of it so we can make sure to not add unnecessary or potentially wrong API.