Avoid allocating uvBuffer on each upscale call

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

@@ -692,16 +692,17 @@ namespace SixLabors.ImageSharp.Formats.Webp.Lossy
             int mbw = io.MbW;
             int uvw = (mbw + 1) / 2;
             int y = io.MbY;
+            byte[] uvBuffer = new byte[(14 * 32) + 15];
 
             if (y == 0)
             {
                 // First line is special cased. We mirror the u/v samples at boundary.
-                YuvConversion.UpSample(curY, default, curU, curV, curU, curV, dst, default, mbw);
+                YuvConversion.UpSample(curY, default, curU, curV, curU, curV, dst, default, mbw, uvBuffer);
             }
             else
             {
                 // We can finish the left-over line from previous call.
-                YuvConversion.UpSample(tmpYBuffer, curY, topU, topV, curU, curV, buf.Slice(dstStartIdx - bufferStride), dst, mbw);
+                YuvConversion.UpSample(tmpYBuffer, curY, topU, topV, curU, curV, buf.Slice(dstStartIdx - bufferStride), dst, mbw, uvBuffer);
                 numLinesOut++;
             }
 
@@ -714,7 +715,7 @@ namespace SixLabors.ImageSharp.Formats.Webp.Lossy
                 topV = curV;
                 curU = curU.Slice(io.UvStride);
                 curV = curV.Slice(io.UvStride);
-                YuvConversion.UpSample(curY.Slice(io.YStride), curY.Slice(ioStride2), topU, topV, curU, curV, dst.Slice(bufferStride), dst.Slice(bufferStride2), mbw);
+                YuvConversion.UpSample(curY.Slice(io.YStride), curY.Slice(ioStride2), topU, topV, curU, curV, dst.Slice(bufferStride), dst.Slice(bufferStride2), mbw, uvBuffer);
                 curY = curY.Slice(ioStride2);
                 dst = dst.Slice(bufferStride2);
             }
@@ -736,7 +737,7 @@ namespace SixLabors.ImageSharp.Formats.Webp.Lossy
                 // Process the very last row of even-sized picture.
                 if ((yEnd & 1) == 0)
                 {
-                    YuvConversion.UpSample(curY, default, curU, curV, curU, curV, dst.Slice(bufferStride), default, mbw);
+                    YuvConversion.UpSample(curY, default, curU, curV, curU, curV, dst.Slice(bufferStride), default, mbw, uvBuffer);
                 }
             }
 

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

@@ -73,12 +73,12 @@ namespace SixLabors.ImageSharp.Formats.Webp.Lossy
         // we interpolate u/v as:
         //  ([9*a + 3*b + 3*c +   d    3*a + 9*b + 3*c +   d] + [8 8]) / 16
         //  ([3*a +   b + 9*c + 3*d      a + 3*b + 3*c + 9*d]   [8 8]) / 16
-        public static void UpSample(Span<byte> topY, Span<byte> bottomY, Span<byte> topU, Span<byte> topV, Span<byte> curU, Span<byte> curV, Span<byte> topDst, Span<byte> bottomDst, int len)
+        public static void UpSample(Span<byte> topY, Span<byte> bottomY, Span<byte> topU, Span<byte> topV, Span<byte> curU, Span<byte> curV, Span<byte> topDst, Span<byte> bottomDst, int len, byte[] uvBuffer)
         {
 #if SUPPORTS_RUNTIME_INTRINSICS
             if (Sse41.IsSupported)
             {
-                UpSampleSse41(topY, bottomY, topU, topV, curU, curV, topDst, bottomDst, len);
+                UpSampleSse41(topY, bottomY, topU, topV, curU, curV, topDst, bottomDst, len, uvBuffer);
             }
             else
 #endif
@@ -156,10 +156,10 @@ namespace SixLabors.ImageSharp.Formats.Webp.Lossy
         //
         // Then m can be written as
         // m = (k + t + 1) / 2 - (((b^c) & (s^t)) | (k^t)) & 1
-        public static void UpSampleSse41(Span<byte> topY, Span<byte> bottomY, Span<byte> topU, Span<byte> topV, Span<byte> curU, Span<byte> curV, Span<byte> topDst, Span<byte> bottomDst, int len)
+        public static void UpSampleSse41(Span<byte> topY, Span<byte> bottomY, Span<byte> topU, Span<byte> topV, Span<byte> curU, Span<byte> curV, Span<byte> topDst, Span<byte> bottomDst, int len, byte[] uvBuffer)
         {
             const int xStep = 3;
-            byte[] uvBuffer = new byte[(14 * 32) + 15];
+            Array.Clear(uvBuffer, 0, uvBuffer.Length);
             Span<byte> ru = uvBuffer.AsSpan(15);
             Span<byte> rv = ru.Slice(32);