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Fix PSNRHVS computation

Cherry-pick libaom #1362e15990eccab8101305be418528824fb32245 to fix
PSNRHVS calculation.

BUG=webm:1620

Change-Id: Ife7bd8056fcaed06992ad76e8c1ab734c3956ad7

--- a/vpx_dsp/psnrhvs.c

+++ b/vpx_dsp/psnrhvs.c

@@ -144,7 +144,7 @@

    been normalized and then squared." Their CSF matrix (from PSNR-HVS)

    was also constructed from the JPEG matrices. I can not find any obvious

    scheme of normalizing to produce their table, but if I multiply their

-   CSF by 0.38857 and square the result I get their masking table.

+   CSF by 0.3885746225901003 and square the result I get their masking table.

    I have no idea where this constant comes from, but deviating from it

    too greatly hurts MOS agreement.

@@ -152,11 +152,15 @@

    Jaakko Astola, Vladimir Lukin, "On between-coefficient contrast masking

    of DCT basis functions", CD-ROM Proceedings of the Third

    International Workshop on Video Processing and Quality Metrics for Consumer

-   Electronics VPQM-07, Scottsdale, Arizona, USA, 25-26 January, 2007, 4 p.*/

+   Electronics VPQM-07, Scottsdale, Arizona, USA, 25-26 January, 2007, 4 p.

+

+   Suggested in aomedia issue #2363:

+   0.3885746225901003 is a reciprocal of the maximum coefficient (2.573509)

+   of the old JPEG based matrix from the paper. Since you are not using that,

+   divide by actual maximum coefficient. */

   for (x = 0; x < 8; x++)

     for (y = 0; y < 8; y++)

-      mask[x][y] =

-          (_csf[x][y] * 0.3885746225901003) * (_csf[x][y] * 0.3885746225901003);

+      mask[x][y] = (_csf[x][y] / _csf[1][0]) * (_csf[x][y] / _csf[1][0]);

   for (y = 0; y < _h - 7; y += _step) {

     for (x = 0; x < _w - 7; x += _step) {

       int i;