Implemented new spectral buffers allocation

src/ImageSharp/Formats/Jpeg/Components/Decoder/HuffmanScanDecoder.cs

@@ -128,10 +128,13 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder
         {
             if (this.componentsLength == 1)
             {
+                this.frame.AllocateComponents(fullScan: true);
                 this.ParseBaselineDataNonInterleaved();
             }
             else
             {
+                // interleaved baseline is the only place where we can optimize memory footprint via reusing single spectral stride
+                this.frame.AllocateComponents(fullScan: false);
                 this.ParseBaselineDataInterleaved();
             }
         }
@@ -305,6 +308,7 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder
         {
             this.CheckProgressiveData();
 
+            this.frame.AllocateComponents(fullScan: true);
             if (this.componentsLength == 1)
             {
                 this.ParseProgressiveDataNonInterleaved();

src/ImageSharp/Formats/Jpeg/Components/Decoder/JpegComponent.cs

@@ -125,6 +125,18 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder
             {
                 JpegThrowHelper.ThrowBadSampling();
             }
+        }
+
+        public void AllocateSpectral(bool fullScan)
+        {
+            if (this.SpectralBlocks != null)
+            {
+                // this method will be called each scan marker so we need to allocate only once
+                return;
+            }
+
+            int blocksPerLineForMcu = this.Frame.McusPerLine * this.HorizontalSamplingFactor;
+            int blocksPerColumnForMcu = fullScan ? this.Frame.McusPerColumn * this.VerticalSamplingFactor : this.VerticalSamplingFactor;
 
             int totalNumberOfBlocks = blocksPerColumnForMcu * (blocksPerLineForMcu + 1);
             int width = this.WidthInBlocks + 1;

src/ImageSharp/Formats/Jpeg/Components/Decoder/JpegFrame.cs

@@ -104,5 +104,14 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder
                 component.Init();
             }
         }
+
+        public void AllocateComponents(bool fullScan)
+        {
+            for (int i = 0; i < this.ComponentCount; i++)
+            {
+                JpegComponent component = this.Components[i];
+                component.AllocateSpectral(fullScan);
+            }
+        }
     }
 }

src/ImageSharp/Formats/Jpeg/JpegDecoderCore.cs

@@ -921,9 +921,10 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
                 this.Frame.MaxVerticalFactor = maxV;
                 this.ColorSpace = this.DeduceJpegColorSpace();
                 this.Metadata.GetJpegMetadata().ColorType = this.ColorSpace == JpegColorSpace.Grayscale ? JpegColorType.Luminance : JpegColorType.YCbCr;
-                this.Frame.InitComponents();
                 this.ImageSizeInMCU = new Size(this.Frame.McusPerLine, this.Frame.McusPerColumn);
 
+                this.Frame.InitComponents();
+
                 // This can be injected in SOF marker callback
                 this.scanDecoder.InjectFrameData(this.Frame, this);
             }