Reimplementation of loop filter

This version of the loop filter supports non-4:2:0 subsampling and
a fourth plane, as well as changing the filtering order to be more
friendly to hardware implementations.

The filters are applied first to all vertical edges within the
64x64 SB, followed by the top horizontal edge and any internal
horizontal edges. Since filtering is applied on each 4x4 edge
serially, a dependency is created from filtering one block edge
to the next. It would be possible to remove this depencnecy by
building all filtering decisions from the unfiltered
reconstruction data.

Change-Id: I08f3e9683eb7bded8a76651cbc50fc0dfdd05fa7