Added SixLabors.ImageSharp.Drawing to the InternalsVisibleTo.

Author: Dirk Lemstra

src/SixLabors.Core/Properties/AssemblyInfo.cs

@@ -39,4 +39,5 @@
 [assembly: InternalsVisibleTo("SixLabors.Exif")]
 [assembly: InternalsVisibleTo("SixLabors.Fonts")]
 [assembly: InternalsVisibleTo("SixLabors.ImageSharp")]
+[assembly: InternalsVisibleTo("SixLabors.ImageSharp.Drawing")]
 [assembly: InternalsVisibleTo("SixLabors.Shapes")]