fix: Avoid creation of unholded ref of CGColor

Author: dr1rrb

src/Uno.UI/UI/Xaml/Controls/Border/BorderLayerRenderer.iOSmacOS.cs

@@ -527,7 +527,7 @@ private static void CreateImageBrushLayers(CGRect fullArea, CGRect insideArea, C
 			{
 				Frame = fullArea,
 				Mask = fillMask,
-				BackgroundColor = new CGColor(0, 0, 0, 0),
+				BackgroundColor = _transparent,
 				MasksToBounds = true,
 			};
 
@@ -565,7 +565,7 @@ private static void CreateGradientBrushLayers(CGRect fullArea, CGRect insideArea
 			{
 				Frame = fullArea,
 				Mask = fillMask,
-				BackgroundColor = new CGColor(0, 0, 0, 0),
+				BackgroundColor = _transparent,
 				MasksToBounds = true,
 			};