Add a loop to drain buffered frames in the decoder

[wantehchang]

If we use a single decoder instance to decode all tiles in an image grid, it is important to endure there are no buffered frames in the decoder before decoding the next tile with the decoder.

BUG=oss-fuzz:66313

[y-guyon]

The code snippet in the documentation of dav1d_get_picture() is a bit vague so I quickly checked its implementation. It should be fine not to call dav1d_picture_unref(&bufferedFrame); here. It is also unlikely that the dav1d API changes in a way it becomes necessary. On the other hand, calling dav1d_picture_unref(&bufferedFrame); here would not hurt.

[wantehchang]

Thank you for looking into this. It is a common API design that a function outputs nothing on failure. This is why I don't call dav1d_picture_unref(&bufferedFrame) if res is negative. Because of your comment, I checked the documentation. I think we can conclude from the following that a frame is output only when res is 0:

 * @param out Output frame. The caller assumes ownership of the returned
 *            reference.
 *
 * @return
 *         0: Success, and a frame is returned.
 *  DAV1D_ERR(EAGAIN): Not enough data to output a frame. dav1d_send_data()
 *                     should be called with new input.
 *  Other negative DAV1D_ERR codes: Error during decoding or because of invalid
 *                                  passed-in arguments.

[vrabaud]

I confirm this fixes the original bug.