Apply suggestions from code review

Co-authored-by: Günther Foidl <gue@korporal.at>

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

@@ -56,7 +56,7 @@ namespace SixLabors.ImageSharp.Formats.Webp.Lossless
                 {
                     Span<uint> srcSpan = bgra.Slice(y * stride);
                     ref uint inputRef = ref MemoryMarshal.GetReference(srcSpan);
-                    for (int x = 0; x + span <= tileWidth; x += span)
+                    for (int x = 0; x <= tileWidth - span; x += span)
                     {
                         int input0Idx = x;
                         int input1Idx = x + (span / 2);
@@ -101,7 +101,7 @@ namespace SixLabors.ImageSharp.Formats.Webp.Lossless
                 {
                     Span<uint> srcSpan = bgra.Slice(y * stride);
                     ref uint inputRef = ref MemoryMarshal.GetReference(srcSpan);
-                    for (int x = 0; x + span <= tileWidth; x += span)
+                    for (int x = 0; x <= tileWidth - span; x += span)
                     {
                         int input0Idx = x;
                         int input1Idx = x + (span / 2);
@@ -170,7 +170,7 @@ namespace SixLabors.ImageSharp.Formats.Webp.Lossless
                 {
                     Span<uint> srcSpan = bgra.Slice(y * stride);
                     ref uint inputRef = ref MemoryMarshal.GetReference(srcSpan);
-                    for (int x = 0; x + span <= tileWidth; x += span)
+                    for (int x = 0; x <= tileWidth - span; x += span)
                     {
                         int input0Idx = x;
                         int input1Idx = x + (span / 2);
@@ -211,7 +211,7 @@ namespace SixLabors.ImageSharp.Formats.Webp.Lossless
                 {
                     Span<uint> srcSpan = bgra.Slice(y * stride);
                     ref uint inputRef = ref MemoryMarshal.GetReference(srcSpan);
-                    for (int x = 0; x + span <= tileWidth; x += span)
+                    for (int x = 0; x <= tileWidth - span; x += span)
                     {
                         int input0Idx = x;
                         int input1Idx = x + (span / 2);

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

@@ -561,7 +561,7 @@ namespace SixLabors.ImageSharp.Formats.Webp.Lossless
                 return (byte)(lower + (quantization >> 1));
             }
 
-            return (byte)(upper & 0xff);
+            return (byte)upper;
         }
 
         /// <summary>