Rename AVPixFmtDescriptor.nb_channels to nb_components, the new name

is more clear and lexically more consistent with API and
documentation.

Originally committed as revision 20545 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavutil/pixdesc.h

@@ -55,7 +55,7 @@ typedef struct AVComponentDescriptor{
  */
 typedef struct AVPixFmtDescriptor{
     const char *name;
-    uint8_t nb_channels;        ///< The number of components each pixel has, (1-4)
+    uint8_t nb_components;      ///< The number of components each pixel has, (1-4)
 
     /**
      * Amount to shift the luma width right to find the chroma width.