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

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.)