New deblocking filter that smooths block boundaries in an estimated
direction of object orientation.

1. Select the proper direction for deblocking filtering.
Compute abs gradient line by line for the block.
Select the direction with least sum of abs gradient.

2. Apply deblocking filtering for a block along this direction.
Apply directional filtering for Y, U, V planes.

Coding gain on Google test set:

%	  	avg_psnr   ovr_psnr  ssim
lowres  	-0.129 	  -0.136    -0.277
midres  	-0.103    -0.127    -0.188
hdres	  	-0.159    -0.158    -0.173
screen_content  -0.408    -0.397    -0.695

Change-Id: Ie8646dcc163ace5d8faf5e502b38342d885efc30