lavu/fifo: deprecate av_fifo_peek2()

It returns a pointer inside the fifo's buffer, which cannot be safely
used without accessing AVFifoBuffer internals. It is easier and safer to
use av_fifo_generic_peek_at().

libavutil/tests/fifo.c

@@ -30,14 +30,6 @@ int main(void)
     for (i = 0; av_fifo_space(fifo) >= sizeof(int); i++)
         av_fifo_generic_write(fifo, &i, sizeof(int), NULL);
 
-    /* peek at FIFO */
-    n = av_fifo_size(fifo) / sizeof(int);
-    for (i = -n + 1; i < n; i++) {
-        int *v = (int *)av_fifo_peek2(fifo, i * sizeof(int));
-        printf("%d: %d\n", i, *v);
-    }
-    printf("\n");
-
     /* peek_at at FIFO */
     n = av_fifo_size(fifo) / sizeof(int);
     for (i = 0; i < n; i++) {