avcodec: introduce YCoCg colorspace

Non perceptual color model that aims to have an increase effectiveness
in compression like the normal YCbCr while having near-lossless/lossless
mapping to RGB.
Signed-off-by: NLuca Barbato <lu_zero@gentoo.org>

libavcodec/avcodec.h

@@ -500,6 +500,7 @@ enum AVColorSpace{
     AVCOL_SPC_BT470BG     = 5, ///< also ITU-R BT601-6 625 / ITU-R BT1358 625 / ITU-R BT1700 625 PAL & SECAM / IEC 61966-2-4 xvYCC601
     AVCOL_SPC_SMPTE170M   = 6, ///< also ITU-R BT601-6 525 / ITU-R BT1358 525 / ITU-R BT1700 NTSC / functionally identical to above
     AVCOL_SPC_SMPTE240M   = 7,
+    AVCOL_SPC_YCOCG       = 8, ///< Used by Dirac / VC-2 and H.264 FRext, see ITU-T SG16
     AVCOL_SPC_NB             , ///< Not part of ABI
 };