Followup cf5f641: pass $params by reference again

&$params was changed to $params, but hooks.txt still documents
this parameter as being passed by reference, and VipsScaler expects this.

Giuseppe and I suspect this may be what's causing the HHVM segfaults
we're seeing in beta labs.

Change-Id: Ib018f23bc44c247aefc277a0c75ff8577f309ab4