webrtc支持的压缩格式

由于用到WEBRTC输入压缩数据，在此记录支持的输入格式(包括YUV、RGB、H264)

// FourCC codes grouped according to implementation efficiency.

// Primary formats should convert in 1 efficient step.
// Secondary formats are converted in 2 steps.
// Auxilliary formats call primary converters.
enum FourCC {
  // 9 Primary YUV formats: 5 planar, 2 biplanar, 2 packed.
  FOURCC_I420 = FOURCC('I', '4', '2', '0'),
  FOURCC_I422 = FOURCC('I', '4', '2', '2'),
  FOURCC_I444 = FOURCC('I', '4', '4', '4'),
  FOURCC_I411 = FOURCC('I', '4', '1', '1'),
  FOURCC_I400 = FOURCC('I', '4', '0', '0'),
  FOURCC_NV21 = FOURCC('N', 'V', '2', '1'),
  FOURCC_NV12 = FO