Baseline jpegs now clear allocated buffers in the decoding loop

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

@@ -96,6 +96,9 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder
 
             this.scanBuffer = new HuffmanScanBuffer(this.stream);
 
+            bool fullScan = this.frame.Progressive || this.frame.MultiScan;
+            this.frame.AllocateComponents(fullScan);
+
             if (!this.frame.Progressive)
             {
                 this.ParseBaselineData();
@@ -123,14 +126,10 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder
         {
             if (this.ComponentsLength == this.frame.ComponentCount)
             {
-                // interleaved - we can convert spectral data stride by stride
-                this.frame.AllocateComponents(fullScan: false);
                 this.ParseBaselineDataInterleaved();
             }
             else
             {
-                // non-interleaved - each scan contains
-                this.frame.AllocateComponents(fullScan: true);
                 this.ParseBaselineDataNonInterleaved();
             }
         }
@@ -303,7 +302,6 @@ 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

@@ -138,7 +138,9 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder
             int spectralAllocWidth = this.SizeInBlocks.Width;
             int spectralAllocHeight = fullScan ? this.SizeInBlocks.Height : this.VerticalSamplingFactor;
 
-            this.SpectralBlocks = this.memoryAllocator.Allocate2D<Block8x8>(spectralAllocWidth, spectralAllocHeight, AllocationOptions.Clean);
+            // We don't need to clear buffer for stride-by-stride approach
+            AllocationOptions allocOptions = fullScan ? AllocationOptions.Clean : AllocationOptions.None;
+            this.SpectralBlocks = this.memoryAllocator.Allocate2D<Block8x8>(spectralAllocWidth, spectralAllocHeight, allocOptions);
         }
     }
 }

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

@@ -20,6 +20,14 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder
         /// </summary>
         public bool Progressive { get; set; }
 
+        /// <summary>
+        /// Gets or sets a value indicating whether the frame is encoded using multiple scans (SOS markers).
+        /// </summary>
+        /// <remarks>
+        /// This is true for progressive and baseline non-interleaved images.
+        /// </remarks>
+        public bool MultiScan { get; set; }
+
         /// <summary>
         /// Gets or sets the precision.
         /// </summary>