Added a test for incremental codec decodes

tests/Tests/SKCodecTest.cs

@@ -41,6 +41,59 @@ namespace SkiaSharp.Tests
 			}
 		}
 
+		[Test]
+		public void DecodePartialImage ()
+		{
+			// read the data here, so we can fake a throttle/download
+			var path = Path.Combine (PathToImages, "baboon.png");
+			var fileData = File.ReadAllBytes (path);
+			SKColor[] correctBytes;
+			using (var bitmap = SKBitmap.Decode (path)) {
+				correctBytes = bitmap.Pixels;
+			}
+
+			int offset = 0;
+			int maxCount = 1024 * 4;
+
+			// the "download" stream needs some data
+			var downloadStream = new MemoryStream ();
+			downloadStream.Write (fileData, offset, maxCount);
+			downloadStream.Position -= maxCount;
+			offset += maxCount;
+
+			using (var codec = SKCodec.Create (new SKManagedStream (downloadStream))) {
+				var info = new SKImageInfo (codec.Info.Width, codec.Info.Height);
+
+				// the bitmap to be decoded
+				using (var incremental = new SKBitmap (info)) {
+
+					// start decoding
+					IntPtr length;
+					var pixels = incremental.GetPixels (out length);
+					var result = codec.StartIncrementalDecode (info, pixels, info.RowBytes);
+
+					// make sure the start was successful
+					Assert.AreEqual (SKCodecResult.Success, result);
+					result = SKCodecResult.IncompleteInput;
+
+					while (result == SKCodecResult.IncompleteInput) {
+						// decode the rest
+						int rowsDecoded = 0;
+						result = codec.IncrementalDecode (out rowsDecoded);
+
+						// write some more data to the stream
+						maxCount = Math.Min (maxCount, fileData.Length - offset);
+						downloadStream.Write (fileData, offset, maxCount);
+						downloadStream.Position -= maxCount;
+						offset += maxCount;
+					}
+
+					// compare to original
+					CollectionAssert.AreEqual (correctBytes, incremental.Pixels);
+				}
+			}
+		}
+
 		[Test]
 		public void BitmapDecodesCorrectly ()
 		{