v4l2codecs: vp9: Fix duplicating a duplicated picture
Duplicating a picture what was already a dup was leading to a crash. Rename the custom picture flags as HOLDS_BUFFER to make its meaning clear. Then save then ref and store the picture as userdata, so it can be obtained when duplicating. Finally, mark the doplicated as HOLDS_BUFFER to avoid thinking it holds a request.
cc @dwlsalmeida