avformat/mov: Increase support for common encryption.

- Parse schm atom to get different encryption schemes.
- Allow senc atom to appear in track fragments.
- Allow 16-byte IVs.
- Allow constant IVs (specified in tenc).
- Allow only tenc to specify encryption (i.e. no senc/saiz/saio).
- Use sample descriptor to detect clear fragments.

This doesn't support:
- Different sample descriptor holding different encryption info.
  - Only first sample descriptor can be encrypted.
- Encrypted sample groups (i.e. seig).
- Non-'cenc' encryption scheme when using -decryption_key.

Signed-off-by: Jacob Trimble <modmaker@google.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavutil/encryption_info.h

@@ -41,7 +41,7 @@ typedef struct AVSubsampleEncryptionInfo {
  * The size of this struct is not part of the public ABI.
  */
 typedef struct AVEncryptionInfo {
-    /** The fourcc encryption scheme. */
+    /** The fourcc encryption scheme, in big-endian byte order. */
     uint32_t scheme;
 
     /**