[misc] Change b64_* to wrap ffmpeg's base64 encoder/decoder

Using ffmpeg's is probably better than having our own, plus it provides us with
decoded length, which we will need to support artwork via pipe.

src/misc.h

@@ -133,11 +133,11 @@ djb_hash(const void *data, size_t len);
 int64_t
 two_str_hash(const char *a, const char *b);
 
-char *
-b64_decode(const char *b64);
+uint8_t *
+b64_decode(int *dstlen, const char *src);
 
 char *
-b64_encode(const uint8_t *in, size_t len);
+b64_encode(const uint8_t *src, int srclen);
 
 uint64_t
 murmur_hash64(const void *key, int len, uint32_t seed);