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