Fix animation timing

The timing of the final animation frame wasn't being recorded correctly.

webp/anim_encode.go

@@ -21,8 +21,8 @@ import "C"
 import (
 	"errors"
 	"fmt"
-	"time"
 	"image"
+	"time"
 	"unsafe"
 )
 
@@ -82,7 +82,9 @@ func (ae *AnimationEncoder) AddFrame(img image.Image, duration time.Duration) er
 		return errors.New("unsupported image type")
 	}
 
-	timestamp := C.int((duration + ae.duration) / time.Millisecond)
+	timestamp := C.int(ae.duration / time.Millisecond)
+	ae.duration += duration
+
 	if C.WebPAnimEncoderAdd(ae.c, pic, timestamp, nil) == 0 {
 		return fmt.Errorf(
 			"Encoding error: %d - %s",
@@ -91,7 +93,6 @@ func (ae *AnimationEncoder) AddFrame(img image.Image, duration time.Duration) er
 		)
 	}
 
-	ae.duration += duration
 	return nil
 }