Modify release_picture_callback to take a picture instead of a data pointer

Note that this is an API/ABI breaking change and I expect some pushback :)

release_picture_callback is awkward to use in its current form. It provides the raw buf pointer, which might be meaningless to the allocator (e.g. if the allocator didn't use data[0] to store the primary pointer, or if the allocator allocated multiple buffers). Also, the API documentation doesn't document how this buf pointer is derived (which could be fixed with a doc change, but I find this approach preferable).

I find the allocator API easier to use and understand (as a user of dav1d) if alloc_picture_callback and release_picture_callback both have the same API (picture in, picture out). This makes fewer assumptions about how the allocators work, which I think is a good thing.