Tidied high bitdepth variance

Refactored code to match current style on master.
Also fixed a bug where some sse results were
not being shifted for high bitdepth.
Also increased internal bitdepth for variance to
avoid saturation.
Also added rounding for variance when computing
shifted results.

Change-Id: I322bbc1b9abe82c8ef72ab97991720240ddf755c
Changed rounding in high bitdepth

Doing the round before the subtraction
gives PSNR improvements.

Change-Id: Ic6fd16a9df9b9fdc2a4fa4140e8fa0994bd261a0
Improved highbitdepth temporal filter

Increasing the strength when filtering
frames for highbitdepth gives a PSNR gain.

Change-Id: Iee2f776c1ae7b8da2ca4d5e58bb11fd0be2483f4
Added support for 10/12bit y4m

vpxenc now accepts high bitdepth y4m files
vpxdec now produces high bitdepth y4m files
(Only if the bitstream is high bitdepth)
Can force 8bit output via --covert-to-8bit

Change-Id: Ife9fc40772aceed32ba47d7ca81024ed09563721