Add tests for discontiguous buffers for png

src/ImageSharp/Formats/Png/PngDecoder.cs

@@ -2,6 +2,7 @@
 // Licensed under the Apache License, Version 2.0.
 
 using System.IO;
+using SixLabors.ImageSharp.Memory;
 using SixLabors.ImageSharp.PixelFormats;
 
 namespace SixLabors.ImageSharp.Formats.Png
@@ -44,7 +45,19 @@ namespace SixLabors.ImageSharp.Formats.Png
             where TPixel : struct, IPixel<TPixel>
         {
             var decoder = new PngDecoderCore(configuration, this);
-            return decoder.Decode<TPixel>(stream);
+
+            try
+            {
+                return decoder.Decode<TPixel>(stream);
+            }
+            catch (InvalidMemoryOperationException ex)
+            {
+                Size dims = decoder.Dimensions;
+
+                // TODO: use InvalidImageContentException here, if we decide to define it
+                // https://github.com/SixLabors/ImageSharp/issues/1110
+                throw new ImageFormatException($"Can not decode image. Failed to allocate buffers for possibly degenerate dimensions: {dims.Width}x{dims.Height}.", ex);
+            }
         }
 
         /// <inheritdoc/>

src/ImageSharp/Formats/Png/PngDecoderCore.cs

@@ -106,7 +106,7 @@ namespace SixLabors.ImageSharp.Formats.Png
         private int currentRow = Adam7.FirstRow[0];
 
         /// <summary>
-        /// The current number of bytes read in the current scanline
+        /// The current number of bytes read in the current scanline.
         /// </summary>
         private int currentRowBytesRead;
 
@@ -132,18 +132,20 @@ namespace SixLabors.ImageSharp.Formats.Png
             this.ignoreMetadata = options.IgnoreMetadata;
         }
 
+        public Size Dimensions => new Size(this.header.Width, this.header.Height);
+
         /// <summary>
         /// Decodes the stream to the image.
         /// </summary>
         /// <typeparam name="TPixel">The pixel format.</typeparam>
-        /// <param name="stream">The stream containing image data. </param>
+        /// <param name="stream">The stream containing image data.</param>
         /// <exception cref="ImageFormatException">
         /// Thrown if the stream does not contain and end chunk.
         /// </exception>
         /// <exception cref="ArgumentOutOfRangeException">
         /// Thrown if the image is larger than the maximum allowable size.
         /// </exception>
-        /// <returns>The decoded image</returns>
+        /// <returns>The decoded image.</returns>
         public Image<TPixel> Decode<TPixel>(Stream stream)
             where TPixel : struct, IPixel<TPixel>
         {