Added bindings for SkOverdrawCanvas

- also set flags to reduce file sizes

binding/Binding/SKOverdrawCanvas.cs

@@ -0,0 +1,22 @@
+using System;
+
+namespace SkiaSharp
+{
+	public class SKOverdrawCanvas : SKNWayCanvas
+	{
+		[Preserve]
+		internal SKOverdrawCanvas (IntPtr handle, bool owns)
+			: base (handle, owns)
+		{
+		}
+
+		public SKOverdrawCanvas (SKCanvas canvas)
+			: this (IntPtr.Zero, true)
+		{
+			if (canvas == null)
+				throw new ArgumentNullException (nameof (canvas));
+
+			Handle = SkiaApi.sk_overdraw_canvas_new (canvas.Handle);
+		}
+	}
+}

binding/Binding/SkiaApi.cs

@@ -7,6 +7,7 @@ using GRBackendObject = System.IntPtr;
 using sk_surface_t = System.IntPtr;
 using sk_canvas_t = System.IntPtr;
 using sk_nodraw_canvas_t = System.IntPtr;
+using sk_overdraw_canvas_t = System.IntPtr;
 using sk_nway_canvas_t = System.IntPtr;
 using sk_image_t = System.IntPtr;
 using sk_paint_t = System.IntPtr;
@@ -317,6 +318,12 @@ namespace SkiaSharp
 		[DllImport(SKIA, CallingConvention = CallingConvention.Cdecl)]
 		public extern static void sk_nway_canvas_remove_all(sk_nway_canvas_t t);
 
+		// overdraw canvas
+		[DllImport(SKIA, CallingConvention = CallingConvention.Cdecl)]
+		public extern static sk_overdraw_canvas_t sk_overdraw_canvas_new(sk_canvas_t canvas);
+		[DllImport(SKIA, CallingConvention = CallingConvention.Cdecl)]
+		public extern static void sk_overdraw_canvas_destroy(sk_overdraw_canvas_t canvas);
+
 		// paint
 		[DllImport(SKIA, CallingConvention = CallingConvention.Cdecl)]
 		public extern static sk_paint_t sk_paint_new();

externals/skia

@@ -1 +1 @@
-Subproject commit 65cfd58f44c312e418a73551850f467a27ab4acf
+Subproject commit d42756a5d583b02ee472e8c747a905c31e2c5f2b

tests/Tests/SKCanvasTest.cs

@@ -91,6 +91,25 @@ namespace SkiaSharp.Tests
 			}
 		}
 
+		[SkippableFact]
+		public void OverdrawCanvasDrawsProperly()
+		{
+			using (var bitmap = new SKBitmap(new SKImageInfo(100, 100)))
+			using (var canvas = new SKCanvas(bitmap))
+			using (var overdraw = new SKOverdrawCanvas(canvas))
+			{
+				bitmap.Erase(SKColors.Transparent);
+
+				overdraw.DrawRect(SKRect.Create(10, 10, 30, 30), new SKPaint());
+				overdraw.DrawRect(SKRect.Create(20, 20, 30, 30), new SKPaint());
+
+				Assert.Equal(0, bitmap.GetPixel(5, 5).Alpha);
+				Assert.Equal(1, bitmap.GetPixel(15, 15).Alpha);
+				Assert.Equal(2, bitmap.GetPixel(25, 25).Alpha);
+				Assert.Equal(1, bitmap.GetPixel(45, 45).Alpha);
+			}
+		}
+
 		[SkippableFact]
 		public void SvgCanvasSavesFile()
 		{