libavformat/crypto - encourage reads of 4096 bytes

the current implementation reads in chunks of 149x16=2384 bytes.
Seems more logical for it to read in chunks of 4096
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/crypto.c

@@ -26,7 +26,8 @@
 #include "internal.h"
 #include "url.h"
 
-#define MAX_BUFFER_BLOCKS 150
+// encourage reads of 4096 bytes - 1 block is always retained.
+#define MAX_BUFFER_BLOCKS 257
 #define BLOCKSIZE 16
 
 typedef struct CryptoContext {