Relax criteria for Shuffle3

src/ImageSharp/Common/Helpers/Shuffle/IComponentShuffle.cs

@@ -28,6 +28,10 @@ namespace SixLabors.ImageSharp
         /// </summary>
         /// <param name="source">The source span of bytes.</param>
         /// <param name="dest">The destination span of bytes.</param>
+        /// <remarks>
+        /// Implementation can assume that source.Length is less or equal than dest.Length.
+        /// Loops should iterate using source.Length.
+        /// </remarks>
         void RunFallbackShuffle(ReadOnlySpan<byte> source, Span<byte> dest);
     }
 

src/ImageSharp/Common/Helpers/SimdUtils.Shuffle.cs

@@ -77,6 +77,7 @@ namespace SixLabors.ImageSharp
             TShuffle shuffle)
             where TShuffle : struct, IShuffle3
         {
+            // Source length should be smaller than dest length, and divisible by 3.
             VerifyShuffle3SpanInput(source, dest);
 
 #if SUPPORTS_RUNTIME_INTRINSICS
@@ -182,9 +183,9 @@ namespace SixLabors.ImageSharp
             where T : struct
         {
             DebugGuard.IsTrue(
-                source.Length == dest.Length,
+                source.Length <= dest.Length,
                 nameof(source),
-                "Input spans must be of same length!");
+                "Source should fit into dest!");
 
             DebugGuard.IsTrue(
                 source.Length % 3 == 0,