pixdesc: put components always in the same order.

libavutil/avutil.h

@@ -153,7 +153,7 @@
  */
 
 #define LIBAVUTIL_VERSION_MAJOR 51
-#define LIBAVUTIL_VERSION_MINOR 43
+#define LIBAVUTIL_VERSION_MINOR 44
 #define LIBAVUTIL_VERSION_MICRO 100
 
 #define LIBAVUTIL_VERSION_INT   AV_VERSION_INT(LIBAVUTIL_VERSION_MAJOR, \

libavutil/pixdesc.h

@@ -76,9 +76,12 @@ typedef struct AVPixFmtDescriptor{
     uint8_t flags;
 
     /**
-     * Parameters that describe how pixels are packed. If the format
-     * has chroma components, they must be stored in comp[1] and
-     * comp[2].
+     * Parameters that describe how pixels are packed.
+     * If the format has 2 or 4 components, then alpha is last.
+     * If the format has 1 or 2 components, then luma is 0.
+     * If the format has 3 or 4 components,
+     * if the RGB flag is set then 0 is red, 1 is green and 2 is blue;
+     * otherwise 0 is luma, 1 is chroma-U and 2 is chroma-V.
      */
     AVComponentDescriptor comp[4];
 }AVPixFmtDescriptor;