Add fast path scaling for YUY2 and other packed YUV formats. Add a new
method to merge the scalers of the Y and UV components into one scaler.
Add faster horizontal 2tap scaler.
See https://bugzilla.gnome.org/show_bug.cgi?id=741987
Add a video scaler object build on top of the resampler. It has
implementation to deal with interlaced video as well as horizontal and
vertical scaling functions.