dav1d.h: group debug options inside their own struct within Dav1dSettings

Should help the user know they can be ignored for normal decoding operation.

It's technically an API break, but the only field affected was added a few days ago and is not present in any release, so it should be ok.

added api-change minor labels

approved this merge request

Dropping this as it would require padding above and under the struct (so we can add fields inside and outside it), and then we need to somehow take alignment into account.

closed