Refactors the switchable filter search in the rd loop to
improve encode speed.

Uses a piecewise approximation to a closed form expression to estimate
rd cost for a Laplacian source with a given variance and quantization
step-size.

About 40% encode time reduction is achieved.

Results (on a feb 12 baseline) show a slight drop:

derf: -0.019%
yt: +0.010%
std-hd: -0.162%
hd: -0.050%

Change-Id: Ie861badf5bba1e3b1052e29a0ef1b7e256edbcd0