Moved IDrawingContextImpl to Avalonia.Platform.

So it can be alongside its impl brethren.

src/Avalonia.Visuals/Media/DrawingContext.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using Avalonia.Media.Imaging;
+using Avalonia.Platform;
 
 namespace Avalonia.Media
 {

src/Avalonia.Visuals/Media/IDrawingContextImpl.cs → src/Avalonia.Visuals/Platform/IDrawingContextImpl.cs

@@ -2,9 +2,10 @@
 // Licensed under the MIT license. See licence.md file in the project root for full license information.
 
 using System;
+using Avalonia.Media;
 using Avalonia.Platform;
 
-namespace Avalonia.Media
+namespace Avalonia.Platform
 {
     /// <summary>
     /// Defines the interface through which drawing occurs.

tests/Avalonia.Visuals.UnitTests/RenderTests_Culling.cs

@@ -6,6 +6,7 @@ using Avalonia.Controls;
 using Avalonia.Media;
 using Avalonia.Rendering;
 using Xunit;
+using Avalonia.Platform;
 
 namespace Avalonia.Visuals.UnitTests
 {

tests/Avalonia.Visuals.UnitTests/VisualTree/BoundsTrackerTests.cs

@@ -13,6 +13,7 @@ using Xunit;
 using Avalonia.Media;
 using Moq;
 using Avalonia.UnitTests;
+using Avalonia.Platform;
 
 namespace Avalonia.Visuals.UnitTests.VisualTree
 {

tests/Avalonia.Visuals.UnitTests/VisualTree/VisualExtensionsTests_GetVisualsAt.cs

@@ -6,6 +6,7 @@ using Avalonia.Controls;
 using Avalonia.Controls.Presenters;
 using Avalonia.Layout;
 using Avalonia.Media;
+using Avalonia.Platform;
 using Avalonia.Rendering;
 using Avalonia.UnitTests;
 using Avalonia.VisualTree;