Track updates to the proposed Matroska Colour spec.

https://mailarchive.ietf.org/arch/msg/cellar/cCjxCohD-2xM93ijoFVaYYMLFas

Changes:
- ChromaSubsampling replaced by ChromaSubsamplingHorz,
  and ChromaSubsamplingVert.
- CbSubsamplingHorz and CbSubsamplingVert added.

Change-Id: I0848f9f41b3162afb5485b82704620d9e03f9b5e

webmids.hpp

@@ -99,14 +99,17 @@ enum MkvId {
   kMkvColour = 0x55B0,
   kMkvMatrix = 0x55B1,
   kMkvBitsPerChannel = 0x55B2,
-  kMkvChromaSubsampling = 0x55B3,
-  kMkvChromaSitingHorz = 0x55B4,
-  kMkvChromaSitingVert = 0x55B5,
-  kMkvRange = 0x55B6,
-  kMkvTransferFunction = 0x55B7,
-  kMkvPrimaries = 0x55B8,
-  kMkvMaxCLL = 0x55B9,
-  kMkvMaxFALL = 0x55BA,
+  kMkvChromaSubsamplingHorz = 0x55B3,
+  kMkvChromaSubsamplingVert = 0x55B4,
+  kMkvCbSubsamplingHorz = 0x55B5,
+  kMkvCbSubsamplingVert = 0x55B6,
+  kMkvChromaSitingHorz = 0x55B7,
+  kMkvChromaSitingVert = 0x55B8,
+  kMkvRange = 0x55B9,
+  kMkvTransferFunction = 0x55BA,
+  kMkvPrimaries = 0x55BB,
+  kMkvMaxCLL = 0x55BC,
+  kMkvMaxFALL = 0x55BD,
   // mastering metadata
   kMkvMasteringMetadata = 0x55D0,
   kMkvPrimaryRChromaticityX = 0x55D1,