Made some types public for 3rd-party Xamarin.Forms platforms (#522)

source/SkiaSharp.Views.Forms/SkiaSharp.Views.Forms.Shared/RendererTypes.cs

@@ -2,7 +2,7 @@
 
 namespace SkiaSharp.Views.Forms
 {
-	internal class GetPropertyValueEventArgs<T> : EventArgs
+	public class GetPropertyValueEventArgs<T> : EventArgs
 	{
 		public T Value { get; set; }
 	}

source/SkiaSharp.Views.Forms/SkiaSharp.Views.Forms.Shared/SKCanvasView.cs

@@ -95,7 +95,7 @@ namespace SkiaSharp.Views.Forms
 		}
 	}
 
-	internal interface ISKCanvasViewController : IViewController
+	public interface ISKCanvasViewController : IViewController
 	{
 		// the native listens to this event
 		event EventHandler SurfaceInvalidated;