audio: skip samples and adjust timestamps ourselves

This gets rid of this warning:

  Could not update timestamps for skipped samples.

This required an API addition to FFmpeg (otherwise it would instead
doing arithmetic on the timestamps itself), so whether it works depends
on the FFmpeg version.

old-configure

@@ -828,6 +828,12 @@ api_statement_check \
     libavutil/frame.h \
     'av_frame_get_metadata(NULL)'
 
+api_statement_check \
+    "libavutil AVFrame skip samples metadata" \
+    HAVE_AVFRAME_SKIP_SAMPLES \
+    libavutil/frame.h \
+    'enum AVFrameSideDataType type = AV_FRAME_DATA_SKIP_SAMPLES'
+
 api_statement_check \
     "libavutil QP API" \
     HAVE_AVUTIL_QP_API \