Reduce allocation when reading PNG chunks

Previously the code would read a byte[] for every PNG chunk, irrespective of whether it was going to be stored. With this change, these arrays are only allocated when they will actually be needed.

This is the .NET port of drewnoakes/metadata-extractor#535

MetadataExtractor/Formats/Png/PngChunkReader.cs

@@ -48,6 +48,11 @@ public IEnumerable<PngChunk> Extract(SequentialReader reader, ICollection<PngChu
             //     Miscellaneous information: bKGD, hIST, pHYs, sPLT
             //     Time information:          tIME
             //
+            // CHUNK READING
+            //
+            //   Only chunk data for types specified in desiredChunkTypes is extracted.
+            //   For empty chunk type list NO data is copied from source stream.
+            //   For null chunk type list ALL data is copied from source stream.
 
             // network byte order
             reader = reader.WithByteOrder(isMotorolaByteOrder: true);
@@ -68,7 +73,17 @@ public IEnumerable<PngChunk> Extract(SequentialReader reader, ICollection<PngChu
                     throw new PngProcessingException("PNG chunk length exceeds maximum");
                 var chunkType = new PngChunkType(reader.GetBytes(4));
                 var willStoreChunk = desiredChunkTypes == null || desiredChunkTypes.Contains(chunkType);
-                var chunkData = reader.GetBytes(chunkDataLength);
+
+                byte[]? chunkData;
+                if (willStoreChunk)
+                {
+                    chunkData = reader.GetBytes(chunkDataLength);
+                }
+                else
+                {
+                    chunkData = null;
+                    reader.Skip(chunkDataLength);
+                }
 
                 // Skip the CRC bytes at the end of the chunk
                 // TODO consider verifying the CRC value to determine if we're processing bad data
@@ -85,7 +100,8 @@ public IEnumerable<PngChunk> Extract(SequentialReader reader, ICollection<PngChu
                 if (chunkType.Equals(PngChunkType.IEND))
                     seenImageTrailer = true;
 
-                if (willStoreChunk)
+                // chunkData will be null if we aren't interested in this chunk
+                if (chunkData is not null)
                     chunks.Add(new PngChunk(chunkType, chunkData));
 
                 seenChunkTypes.Add(chunkType);