libvpx

Enable dual buffer rd search and encoding scheme

This commit enables the dual buffer rate-distortion optimization
and encoding scheme. It stacks the original transform coefficients,
quantized levels, and reconstructed coefficients, in the rate-
distortion optimization search process, hence eliminates the need
to re-run residual generation, forward transform, and quantization
in the encoding stage.

Change-Id: I011bfad3a59a380a869ee552e91dae0394ec492e