libvpx

Reversible WHT pair

This commit changed the forward and the inverse 4x4 Walsh Hadamard
transform to a new pair, where the inverse transform can pefectly
reconstuct the input to forward transform. It also does so without
changing the input and output value range. Even more, it does not
change the complexity of the transforms.

While it was not expected to improve the results of our current test,
it does improve std-hd set by 0.2% on all metrics. No change on derf.

Change-Id: Ie4f23ddd3a0f3c5fbe97fb58399f860031f99337