encode FrameDuration

Author: Poker-sang

src/ImageSharp/Formats/Webp/Lossless/Vp8LEncoder.cs

@@ -307,7 +307,8 @@ public void Encode<TPixel>(ImageFrame<TPixel> frame, Stream stream, bool hasAnim
             prevPosition = BitWriterBase.WriteAnimationFrame(stream, new()
             {
                 Width = (uint)frame.Width,
-                Height = (uint)frame.Height
+                Height = (uint)frame.Height,
+                Duration = frame.Metadata.GetWebpMetadata().FrameDuration
             });
         }
 

src/ImageSharp/Formats/Webp/Lossy/Vp8Encoder.cs

@@ -477,7 +477,8 @@ private void Encode<TPixel>(ImageFrame<TPixel> frame, Stream stream, bool hasAni
                 prevPosition = BitWriterBase.WriteAnimationFrame(stream, new()
                 {
                     Width = (uint)frame.Width,
-                    Height = (uint)frame.Height
+                    Height = (uint)frame.Height,
+                    Duration = frame.Metadata.GetWebpMetadata().FrameDuration
                 });
             }