Force the non-opacity of the SKCanvasView on iOS

On Android and UWP, this view defaults to a transparent background, whereas
on iOS it was opaque by default. This patch fixes it in the name of consistency.

source/SkiaSharp.Views.Forms/SkiaSharp.Views.Forms.iOS/SKCanvasViewRenderer.cs

@@ -86,6 +86,9 @@ namespace SkiaSharp.Views.Forms
 				UserInteractionEnabled = false;
 
 				this.controller = controller;
+
+				// Force the opacity to false for consistency with the other platforms
+				Opaque = false;
 			}
 
 			public override void DrawInSurface(SKSurface surface, SKImageInfo info)