Use more accurate IDCT

src/ImageSharp/Formats/Jpeg/PdfJsPort/PdfJsJpegDecoderCore.cs

@@ -786,7 +786,9 @@ namespace ImageSharp.Formats.Jpeg.PdfJsPort
             int blocksPerLine = component.BlocksPerLine;
             int blocksPerColumn = component.BlocksPerColumn;
             using (var computationBuffer = Buffer<short>.CreateClean(64))
-            using (var multiplicationBuffer = Buffer<short>.CreateClean(64))
+
+            // IDCTFast Only
+            // using (var multiplicationBuffer = Buffer<short>.CreateClean(64))
             {
                 Span<short> quantizationTable =
                     this.quantizationTables.Tables.GetRowSpan(frameComponent.QuantizationIdentifier);
@@ -797,22 +799,30 @@ namespace ImageSharp.Formats.Jpeg.PdfJsPort
                 //   scalefactor[0] = 1
                 //   scalefactor[k] = cos(k*PI/16) * sqrt(2)    for k=1..7
                 // For integer operation, the multiplier table is to be scaled by 12.
-                Span<short> multiplierSpan = multiplicationBuffer;
-                for (int i = 0; i < 64; i++)
-                {
-                    multiplierSpan[i] = (short)PdfJsIDCT.Descale(quantizationTable[i] * PdfJsIDCT.Aanscales[i], 12);
-                }
 
+                // IDCTFast Only
+                // Span<short> multiplierSpan = multiplicationBuffer;
+                // for (int i = 0; i < 64; i++)
+                // {
+                //    multiplierSpan[i] = (short)PdfJsIDCT.Descale(quantizationTable[i] * PdfJsIDCT.Aanscales[i], 12);
+                // }
                 for (int blockRow = 0; blockRow < blocksPerColumn; blockRow++)
                 {
                     for (int blockCol = 0; blockCol < blocksPerLine; blockCol++)
                     {
                         int offset = GetBlockBufferOffset(ref component, blockRow, blockCol);
-                        PdfJsIDCT.QuantizeAndInverseFast(
+
+                        // IDCTFast Only
+                        // PdfJsIDCT.QuantizeAndInverseFast(
+                        //    ref frameComponent,
+                        //    offset,
+                        //    ref computationBufferSpan,
+                        //    ref multiplierSpan);
+                        PdfJsIDCT.QuantizeAndInverse(
                             ref frameComponent,
                             offset,
                             ref computationBufferSpan,
-                            ref multiplierSpan);
+                            ref quantizationTable);
                     }
                 }
             }