[C# API] Corrected the scale/translation for iOS

- scale the canvas for smoothing
 - flip the image for CGContext

samples/Skia.Forms.Demo/iOS/NativeSkiaView.cs

@@ -1,4 +1,5 @@
 using System;
+using CoreGraphics;
 using UIKit;
 using SkiaSharp;
 
@@ -6,6 +7,8 @@ namespace Skia.Forms.Demo.iOS
 {
 	public class NativeSkiaView: UIView
 	{
+		const int bitmapInfo = ((int)CGBitmapFlags.ByteOrder32Big) | ((int)CGImageAlphaInfo.PremultipliedLast);
+
 		ISkiaViewController skiaView;
 
 		public NativeSkiaView (SkiaView skiaView)
@@ -13,37 +16,32 @@ namespace Skia.Forms.Demo.iOS
 			this.skiaView = skiaView;
 		}
 
-		public override void Draw (CoreGraphics.CGRect rect)
+		public override void Draw (CGRect rect)
 		{
 			base.Draw (rect);
 
-			// Just not that sharp using the scale pixel scale only
-			// Going going 2x Pixel Scale
-			var screenScale = (int)UIScreen.MainScreen.Scale * 2;
-			var width = (int)Bounds.Width * screenScale;
-			var height = (int)Bounds.Height * screenScale;
+			var screenScale = UIScreen.MainScreen.Scale;
+			var width = (int)(Bounds.Width * screenScale);
+			var height = (int)(Bounds.Height * screenScale);
 
 			IntPtr buff = System.Runtime.InteropServices.Marshal.AllocCoTaskMem (width * height * 4);
 			try {
-
-				using (var surface = SKSurface.Create (width, height, SKColorType.Rgba_8888, SKAlphaType.Premul, buff, width * 4)) {
+				using (var surface = SKSurface.Create (width, height, SKColorType.N_32, SKAlphaType.Premul, buff, width * 4)) {
 					var skcanvas = surface.Canvas;
-
-					// 2 for one here.  Scaling to the pixel size + moving the origin to the top left
-					skcanvas.Scale (screenScale, -screenScale);
-					skcanvas.Translate (0, (float)-Frame.Height);
-					skiaView.SendDraw (skcanvas);
+					skcanvas.Scale ((float)screenScale, (float)screenScale);
+					using (new SKAutoCanvasRestore (skcanvas, true)) {
+						skiaView.SendDraw (skcanvas);
+					}
 				}
 
-				using (var colorSpace = CoreGraphics.CGColorSpace.CreateDeviceRGB ()) {
-					int hack = ((int)CoreGraphics.CGBitmapFlags.ByteOrderDefault) | ((int)CoreGraphics.CGImageAlphaInfo.PremultipliedLast);
-					using (var bContext = new CoreGraphics.CGBitmapContext (buff, width, height, 8, width * 4, colorSpace, (CoreGraphics.CGImageAlphaInfo) hack)) {
-						using (var image = bContext.ToImage ()) {
-							using (var context = UIGraphics.GetCurrentContext ()) {
-								context.DrawImage (Bounds, image);
-							}
-						}
-					}
+				using (var colorSpace = CGColorSpace.CreateDeviceRGB ())
+				using (var bContext = new CGBitmapContext (buff, width, height, 8, width * 4, colorSpace, (CGImageAlphaInfo)bitmapInfo))
+				using (var image = bContext.ToImage ())
+				using (var context = UIGraphics.GetCurrentContext ()) {
+					// flip the image for CGContext.DrawImage
+					context.TranslateCTM (0, Frame.Height);
+					context.ScaleCTM (1, -1);
+					context.DrawImage (Bounds, image);
 				}
 			} finally {
 				if (buff != IntPtr.Zero)