api: simplistic auto-determination of frame thread count

--HG--
extra : amend_source : f41b4be60f6972a5d380acf92632be0e16028c52
api: output x265_picture.bitDepth should reflect actual internal bitdepth

And not sizeof(pixel) * 8.
tcompicyuv: fix for copyFromPicture() when HIGH_BIT_DEPTH enable, sizeof(Pel)=2 and pic.bitDepth=8