Implemented Skia DPI scaling.

Has a workaround for bug in DrawingContext.

src/Skia/Avalonia.Skia.Desktop/RenderTarget.cs

@@ -79,6 +79,31 @@ namespace Avalonia.Skia
 #endif
         }
 
+#if WIN32
+        private Size GetWindowDpi()
+        {
+            if (UnmanagedMethods.ShCoreAvailable)
+            {
+                uint dpix, dpiy;
+
+                var monitor = UnmanagedMethods.MonitorFromWindow(
+                    _hwnd.Handle,
+                    UnmanagedMethods.MONITOR.MONITOR_DEFAULTTONEAREST);
+
+                if (UnmanagedMethods.GetDpiForMonitor(
+                        monitor,
+                        UnmanagedMethods.MONITOR_DPI_TYPE.MDT_EFFECTIVE_DPI,
+                        out dpix,
+                        out dpiy) == 0)
+                {
+                    return new Size(dpix, dpiy);
+                }
+            }
+
+            return new Size(96, 96);
+        }
+#endif
+
         public override DrawingContext CreateDrawingContext()
         {
             FixSize();
@@ -89,9 +114,16 @@ namespace Avalonia.Skia
             canvas.Clear(SKColors.Red);
             canvas.ResetMatrix();
 
+            double scale = 1.0;
+
+#if WIN32
+            var dpi = GetWindowDpi();
+            scale = dpi.Width / 96.0;
+#endif
+
             return
                 new DrawingContext(
-                    new WindowDrawingContextImpl(this));
+                    new WindowDrawingContextImpl(this, scale));
         }
 
         public void Present()

src/Skia/Avalonia.Skia/DrawingContextImpl.cs

@@ -11,13 +11,19 @@ namespace Avalonia.Skia
     internal class DrawingContextImpl : IDrawingContextImpl
     {
         private Stack<PaintWrapper> maskStack = new Stack<PaintWrapper>();
-        
+        private Matrix _dpiScaleMatrix;
+        private Matrix _currentTransform = Matrix.Identity;
+
         public SKCanvas Canvas { get; private set; }
 
-        public DrawingContextImpl(SKCanvas canvas)
+        public DrawingContextImpl(SKCanvas canvas, double scale = 1.0)
         {
             Canvas = canvas;
             Canvas.Clear();
+
+            _dpiScaleMatrix = Matrix.CreateScale(scale, scale);
+
+            _currentTransform =  Matrix.Identity * _dpiScaleMatrix;
         }
 
         public void DrawImage(IBitmap source, double opacity, Rect sourceRect, Rect destRect)
@@ -342,9 +348,7 @@ namespace Avalonia.Skia
             }
             Canvas.Restore();
             Canvas.Restore();
-        }
-
-        private Matrix _currentTransform = Matrix.Identity;
+        }        
 
         public Matrix Transform
         {
@@ -355,7 +359,11 @@ namespace Avalonia.Skia
                     return;
 
                 _currentTransform = value;
-                Canvas.SetMatrix(value.ToSKMatrix());
+
+                // TODO this line is a workaround for bug in DrawingContext.cs
+                _currentTransform *= _dpiScaleMatrix;
+
+                Canvas.SetMatrix(_currentTransform.ToSKMatrix());
             }
         }
     }

src/Skia/Avalonia.Skia/WindowDrawingContextImpl.cs

@@ -6,8 +6,8 @@ namespace Avalonia.Skia
     {
         WindowRenderTarget _target;
 
-        public WindowDrawingContextImpl(WindowRenderTarget target)
-            : base(target.Surface.Canvas)
+        public WindowDrawingContextImpl(WindowRenderTarget target, double scale = 1.0)
+            : base(target.Surface.Canvas, scale)
         {
             _target = target;
         }