libvpx

move LFWorkerData allocation to VP9LfSync

this removes an assumption that worker->data1 would be pointing to a
TileWorkerData allocation.
additionally, within the multi-threaded loopfilter pass VP9LfSync as a
parameter to the worker hook, removing the need for a shadow pointer in
LFWorkerData.

Change-Id: Ic7b2faa34e3eb59dbcb8a7c67f333448fa047c88