Add av_log_{ask_for_sample|missing_feature} replacements to libavutil

This allows reporting missing features and requesting samples from
all libraries in a standard way; with a simplified API.
av_log_missing_feature() ---> avpriv_report_missing_feature()
avformat: av_log_ask_for_sample() ---> avpriv_request_sample()
rsodec: Use avpriv_report_missing_feature() where appropriate
avcodec: av_log_ask_for_sample() ---> avpriv_request_sample()
avcodec: av_log_missing_feature(1) ---> avpriv_request_sample()
vaapi: fix argument for ff_vaapi_common_end_frame call