avcodec: add a subcharenc mode that disables UTF-8 check

This is for applications which want to explicitly check for invalid
UTF-8 manually, and take actions that are better than dropping invalid
subtitles silently. (It's pretty much silent because sporadic avcodec
error messages are so common that you can't reasonably display them in a
prominent and meaningful way in a application GUI.)

doc/APIchanges

@@ -15,6 +15,9 @@ libavutil:     2017-10-21
 
 API changes, most recent first:
 
+2018-03-xx - xxxxxxx - lavc 58.16.100 - avcodec.h
+  Add FF_SUB_CHARENC_MODE_IGNORE.
+
 2018-xx-xx - xxxxxxx - lavu 56.8.100 - encryption_info.h
   Add AVEncryptionInitInfo and AVEncryptionInfo structures to hold new side-data
   for encryption info.