libhb: Add libavfilter support and pad filter

libhb: Add libavfilter support and pad filter

New filter types HB_FILTER_AVFILTER and HB_FILTER_PAD.
Settings for HB_FILTER_AVFILTER are the same as you would pass to avconv
from the command line -vf option, except that we do not support
multi-input or multi-output filters.

Settings for HB_FILTER_PAD are "width:height:color:x_offset:y_offset".
width x height is the size of the output frame after padding.
color may be a w3c color name or RGB value (default black).
x_offset, y_offset is the position of the video within the padded area
(default centered).
Any of the values may be omitted or "auto".