Add option to extract keyframe only during trickplay image generation

This would be significantly faster than decoding every frame, but it does have compatibility issues. Not all decoders support this mode, notably the VP9 decoder, CUVID decoders, and QSV decoders.

Some videos with very long key-frame intervals may also perform poorly with this mode, as the image timing could become too inaccurate to reflect the actual frame.

Signed-off-by: gnattu <gnattuoc@me.com>

MediaBrowser.Controller/MediaEncoding/IMediaEncoder.cs

@@ -153,6 +153,7 @@ namespace MediaBrowser.Controller.MediaEncoding
         /// <param name="threads">The input/output thread count for ffmpeg.</param>
         /// <param name="qualityScale">The qscale value for ffmpeg.</param>
         /// <param name="priority">The process priority for the ffmpeg process.</param>
+        /// <param name="enableKeyFrameOnlyExtraction">Whether to only extract key frames.</param>
         /// <param name="encodingHelper">EncodingHelper instance.</param>
         /// <param name="cancellationToken">The cancellation token.</param>
         /// <returns>Directory where images where extracted. A given image made before another will always be named with a lower number.</returns>
@@ -168,6 +169,7 @@ namespace MediaBrowser.Controller.MediaEncoding
             int? threads,
             int? qualityScale,
             ProcessPriorityClass? priority,
+            bool enableKeyFrameOnlyExtraction,
             EncodingHelper encodingHelper,
             CancellationToken cancellationToken);