Add missing ITextShaperImpl

src/Headless/Avalonia.Headless/HeadlessPlatformRenderInterface.cs

@@ -20,7 +20,8 @@ namespace Avalonia.Headless
         {
             AvaloniaLocator.CurrentMutable
                 .Bind<IPlatformRenderInterface>().ToConstant(new HeadlessPlatformRenderInterface())
-                .Bind<IFontManagerImpl>().ToConstant(new HeadlessFontManagerStub());
+                .Bind<IFontManagerImpl>().ToConstant(new HeadlessFontManagerStub())
+                .Bind<ITextShaperImpl>().ToConstant(new HeadlessTextShaperStub());
         }
 
         public IEnumerable<string> InstalledFontNames { get; } = new[] { "Tahoma" };