pixel size api impl updated.

src/Avalonia.Native/GlPlatformFeature.cs

@@ -100,12 +100,12 @@ namespace Avalonia.Native
 
         public IGlDisplay Display { get; }
 
-        public System.Drawing.Size PixelSize
+        public PixelSize Size
         {
             get
             {
                 var s = _session.GetPixelSize();
-                return new System.Drawing.Size(s.Width, s.Height);
+                return new PixelSize(s.Width, s.Height);
             }
         }
 

src/Gtk/Avalonia.Gtk3/WindowBaseImpl.cs

@@ -510,12 +510,12 @@ namespace Avalonia.Gtk3
                 : new ImmediateRenderer(root);
         }
 
-        System.Drawing.Size EglGlPlatformSurface.IEglWindowGlPlatformSurfaceInfo.PixelSize
+        PixelSize EglGlPlatformSurface.IEglWindowGlPlatformSurfaceInfo.Size
         {
             get
             {
                 var cs = ClientSize;
-                return new System.Drawing.Size((int)Math.Max(1, LastKnownScaleFactor * cs.Width),
+                return new PixelSize((int)Math.Max(1, LastKnownScaleFactor * cs.Width),
                     (int)Math.Max(1, LastKnownScaleFactor * ClientSize.Height));
             }
         }

src/Windows/Avalonia.Win32/WindowImpl.cs

@@ -933,14 +933,14 @@ namespace Avalonia.Win32
 
             _topmost = value;
         }
-        
-        System.Drawing.Size EglGlPlatformSurface.IEglWindowGlPlatformSurfaceInfo.PixelSize
+
+        PixelSize EglGlPlatformSurface.IEglWindowGlPlatformSurfaceInfo.Size
         {
             get
             {
                 RECT rect;
                 GetClientRect(_hwnd, out rect);
-                return new System.Drawing.Size(
+                return new PixelSize(
                     Math.Max(1, rect.right - rect.left),
                     Math.Max(1, rect.bottom - rect.top));
             }