Make D2D backend match Skia.

Skia doesn't support getting DPI from loaded bitmaps and always returns 96. Do the same in D2D.

src/Windows/Avalonia.Direct2D1/Media/Imaging/D2DBitmapImpl.cs

@@ -30,7 +30,7 @@ namespace Avalonia.Direct2D1.Media
             _direct2DBitmap = d2DBitmap ?? throw new ArgumentNullException(nameof(d2DBitmap));
         }
 
-        public override Vector Dpi => _direct2DBitmap.DotsPerInch.ToAvaloniaVector();
+        public override Vector Dpi => new Vector(96, 96);
         public override PixelSize PixelSize => _direct2DBitmap.PixelSize.ToAvalonia();
 
         public override void Dispose()
@@ -58,3 +58,4 @@ namespace Avalonia.Direct2D1.Media
         }
     }
 }
+;

src/Windows/Avalonia.Direct2D1/Media/Imaging/WicBitmapImpl.cs

@@ -26,6 +26,7 @@ namespace Avalonia.Direct2D1.Media
             using (BitmapDecoder decoder = new BitmapDecoder(Direct2D1Platform.ImagingFactory, fileName, DecodeOptions.CacheOnDemand))
             {
                 WicImpl = new Bitmap(Direct2D1Platform.ImagingFactory, decoder.GetFrame(0), BitmapCreateCacheOption.CacheOnDemand);
+                Dpi = new Vector(96, 96);
             }
         }
 
@@ -39,6 +40,7 @@ namespace Avalonia.Direct2D1.Media
             _decoder = new BitmapDecoder(Direct2D1Platform.ImagingFactory, stream, DecodeOptions.CacheOnLoad);
 
             WicImpl = new Bitmap(Direct2D1Platform.ImagingFactory, _decoder.GetFrame(0), BitmapCreateCacheOption.CacheOnLoad);
+            Dpi = new Vector(96, 96);
         }
 
         /// <summary>
@@ -62,6 +64,7 @@ namespace Avalonia.Direct2D1.Media
                 pixelFormat.Value.ToWic(),
                 BitmapCreateCacheOption.CacheOnLoad);
             WicImpl.SetResolution(dpi.X, dpi.Y);
+            Dpi = dpi;
         }
 
         public WicBitmapImpl(APixelFormat format, IntPtr data, PixelSize size, Vector dpi, int stride)
@@ -70,6 +73,8 @@ namespace Avalonia.Direct2D1.Media
             WicImpl.SetResolution(dpi.X, dpi.Y);
 
             PixelFormat = format;
+            Dpi = dpi;
+
             using (var l = WicImpl.Lock(BitmapLockFlags.Write))
             {
                 for (var row = 0; row < size.Height; row++)
@@ -82,14 +87,7 @@ namespace Avalonia.Direct2D1.Media
             }
         }
 
-        public override Vector Dpi
-        {
-            get
-            {
-                WicImpl.GetResolution(out double x, out double y);
-                return new Vector(x, y);
-            }
-        }
+        public override Vector Dpi { get; }
 
         public override PixelSize PixelSize => WicImpl.Size.ToAvalonia();