Rewrite filter_selectively_horiz for parallel loopfiltering

Added loop filter mask checking, and made the caller function
ready for implementation of parallel loopfiltering in horizontal
direction.

Next, we need to go through the loopfilter functions (both c and
optimized versions), and provide 16-byte wide loopfiltering for
each filter type.

Change-Id: Ifef47e7ef9086ebc2fd6ca7ede8f27c9bbf79e66