Add function to compute skin map for a given block, as its
used in several places (cyclic refresh, noise estimation, and denoising).
Change-Id: Ied622908df43b6927f7fafc6c019d1867f2a24eb
Simple skin detection, from vp8; works reasonable on most of the
RTC clips, but could miss sometimes.
Added debug flag to write out skin map over source input.
Change-Id: I2caea7592f1c459047aac46627eeb24a94946464