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.MediaEncoding/Encoder/MediaEncoder.cs

@@ -807,6 +807,7 @@ namespace MediaBrowser.MediaEncoding.Encoder
             int? threads,
             int? qualityScale,
             ProcessPriorityClass? priority,
+            bool enableKeyFrameOnlyExtraction,
             EncodingHelper encodingHelper,
             CancellationToken cancellationToken)
         {
@@ -862,6 +863,11 @@ namespace MediaBrowser.MediaEncoding.Encoder
                 inputArg = "-threads " + threads + " " + inputArg; // HW accel args set a different input thread count, only set if disabled
             }
 
+            if (enableKeyFrameOnlyExtraction)
+            {
+                inputArg = "-skip_frame nokey " + inputArg;
+            }
+
             var filterParam = encodingHelper.GetVideoProcessingFilterParam(jobState, options, vidEncoder).Trim();
             if (string.IsNullOrWhiteSpace(filterParam))
             {