Clarify non constness of src in av_fifo_generic_write()

Originally committed as revision 21997 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavutil/fifo.c

@@ -78,6 +78,7 @@ int av_fifo_realloc2(AVFifoBuffer *f, unsigned int new_size) {
     return 0;
 }
 
+// src must NOT be const as it can be a context for func that may need updating (like a pointer or byte counter)
 int av_fifo_generic_write(AVFifoBuffer *f, void *src, int size, int (*func)(void*, void*, int))
 {
     int total = size;