Support additional where tuples in ORM_MPTT::parents.  Fixes #1980.
Follow-in to cff1e76e8da2055f9faf7449222b43a686014b1c for #1978
Restrict which album cover ids we swap over to the hierarchy of the
current album, otherwise we can wind up in sticky situations with
hierarchical album cover chains.

Eg, you have a hierarchy like this:

root -> A1 -> A2 --> A3 -> P1
                     A4 -> P2

P1 is the album cover for its entire hierarchy.  But then you
swap A2's album cover for A3 making this:

root -> A1 -> A2 +   A3 -> P1
                 \-> A4 -> P2

Since A1, A2 and A3 all had P1 as their album cover item id.  Now
we're swapping it over to P2 but we want to leave P1 as A3's album
cover item id.  So only look at A4's hierarchy and ignore its peers.