Change the encryption feature to use a callback for decryption.

This allows code calling the library can choose an arbitrary
encryption algorithm.

Decoder control parameter VP8_SET_DECRYPT_KEY is renamed to
VP8D_SET_DECRYPTOR, and now takes an small config struct instead
of just a byte array.

Change-Id: I0462b3388d8d45057e4f79a6b6777fe713dc546e

vp8/decoder/onyxd_int.h

@@ -122,7 +122,8 @@ typedef struct VP8D_COMP
     int independent_partitions;
     int frame_corrupt_residual;
 
-    const unsigned char *decrypt_key;
+    vp8_decrypt_cb *decrypt_cb;
+    void *decrypt_state;
 } VP8D_COMP;
 
 int vp8_decode_frame(VP8D_COMP *cpi);