This function used to set codec_type. With the current fallback
implementation based on avcodec_get_context_defaults3, codec_type
won't be set to the value passed in, but will be set to
AVMEDIA_TYPE_UNKNOWN. Legacy callers of this function might expect
this field to be set to the value passed in.
Signed-off-by: Martin Storsjö <martin@martin.st>