Update three more spans to ReadOnly

src/ImageSharp/Formats/Png/Filters/NoneFilter.cs

@@ -20,7 +20,7 @@ namespace SixLabors.ImageSharp.Formats.Png.Filters
         /// <param name="scanline">The scanline to encode</param>
         /// <param name="result">The filtered scanline result.</param>
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public static void Encode(Span<byte> scanline, Span<byte> result)
+        public static void Encode(ReadOnlySpan<byte> scanline, Span<byte> result)
         {
             // Insert a byte before the data.
             result[0] = 0;

src/ImageSharp/ImageFrame.LoadPixelData.cs

@@ -23,7 +23,7 @@ namespace SixLabors.ImageSharp
         /// <param name="height">The height of the final image.</param>
         /// <typeparam name="TPixel">The pixel format.</typeparam>
         /// <returns>A new <see cref="Image{TPixel}"/>.</returns>
-        public static ImageFrame<TPixel> LoadPixelData<TPixel>(MemoryManager memoryManager, Span<byte> data, int width, int height)
+        public static ImageFrame<TPixel> LoadPixelData<TPixel>(MemoryManager memoryManager, ReadOnlySpan<byte> data, int width, int height)
             where TPixel : struct, IPixel<TPixel>
             => LoadPixelData(memoryManager, MemoryMarshal.Cast<byte, TPixel>(data), width, height);
 
@@ -36,7 +36,7 @@ namespace SixLabors.ImageSharp
         /// <param name="height">The height of the final image.</param>
         /// <typeparam name="TPixel">The pixel format.</typeparam>
         /// <returns>A new <see cref="Image{TPixel}"/>.</returns>
-        public static ImageFrame<TPixel> LoadPixelData<TPixel>(MemoryManager memoryManager, Span<TPixel> data, int width, int height)
+        public static ImageFrame<TPixel> LoadPixelData<TPixel>(MemoryManager memoryManager, ReadOnlySpan<TPixel> data, int width, int height)
             where TPixel : struct, IPixel<TPixel>
         {
             int count = width * height;