expose try_read_decoded_frame to help with real-time use cases

[jbrough]

hey guys, I have a simple use case where I want to pass XcoderDecoder a buffer that is actually a ring buffer (rtrb in this case) rather than a Vec so that I can add frames to the decoder from another thread (eg to support real-time scaling of a live broadcast)

That all works with no library changes due to the flexible iterable trait on the frames input: https://github.com/wavey-ai/xcoder-queue/blob/main/src/lib.rs

But it requires try_read_decoded_frame to be public so the decoder can be saturated with enough initial frames without blocking the producer:

for frame in frames {
    producer_queue.push(frame);
    // these calls will eventually interleave but the decoder needs more than one initial frame to get rollling
    if let Some(decoded_frame) = decoder.try_read_decoded_frame().expect("Failed to read frame") {
        // do something real-time
    }
}

also, I had to make mod linux_imp public to be able to access XcoderPixelFormat in the test (and eventually will be needed in the prod code that uses av-rs) but I feel I may be doing something wrong, there.

(I've tested this tweaked approach with NetINT Quadra on their demo server and its working well, produces a scaled, playable h264 with the decorder+scaler interleaving output frames as I add them to the decoder)