clean up bilinear filter

make reference version of bilinear_filters short.
use reference versions of bilinear_filters and sub_pel_filters when
possible.

recognize that Width was being passed into
filter_block2d_bil_first_pass multiple times. ARM version had already
fixed this. propegate to C.

change references to src_pixels_per_line to src_pitch and standardize on
src/dst (instead of input/output).

recognize that first_pass is only run in the verticle and second_pass
only horizontal. ARM version had already fixed this. propegate to C

Change-Id: I292d376d239a9a7ca37ec2bf03cc0720606983e2