picture: add a flags field

Useful to signal changes in the bitstream the user may want to know about, and in the future could also be used to signal the presence of certain metadata attached to the picture.

Considering these two flags more than signaling something strictly related to the frame in question are signaling a change in the context of the decoding process, another option i thought about was event flags in the decoder itself. But this is harder to implement both as to when it should happen (Should these two specific events be signaled when the frame was parsed/decoded, or when returned? How for the latter?) and how it's exposed to the user (Dav1dContext is opaque, so this would require a function to fetch new events the user would have to query after every call to send_data() and receive_picture()), so I decided this approach was better.