libvpx

Tune rate-distortion modeling to account for frame light change

When the variance is far less than sse, the block is considered to
be under light change. All the energy is compacted into DC coeff
and can be coded at low cost. In such situation, switch the rate-
distortion modeling from sse+var based back to variance based.

Note that this is a temporary solution to handle the rare situations
where the scene light changes.

Change-Id: I1ee0fe2b9eda6b5fac40152e1841bf23f4d229fd