Update ImageSharp reference

Author: JimBobSquarePants

src/ImageSharp.Drawing/ImageSharp.Drawing.csproj

@@ -21,7 +21,7 @@
 
   <ItemGroup>
     <PackageReference Include="SixLabors.Fonts" Version="1.0.0-beta15.24" />
-    <PackageReference Include="SixLabors.ImageSharp" Version="2.0.0-alpha.0.142" />
+    <PackageReference Include="SixLabors.ImageSharp" Version="2.0.0-alpha.0.156" />
   </ItemGroup>
 
   <Import Project="..\..\shared-infrastructure\src\SharedInfrastructure\SharedInfrastructure.projitems" Label="Shared" />