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.Model/Configuration/TrickplayOptions.cs

@@ -18,6 +18,12 @@ public class TrickplayOptions
     /// </summary>
     public bool EnableHwEncoding { get; set; } = false;
 
+    /// <summary>
+    /// Gets or sets a value indicating whether to only extract key frames.
+    /// Significantly faster, but is not compatible with all decoders and/or video files.
+    /// </summary>
+    public bool EnableKeyFrameOnlyExtraction { get; set; } = false;
+
     /// <summary>
     /// Gets or sets the behavior used by trickplay provider on library scan/update.
     /// </summary>