[CoreGraphics] Fix lifetime of delegates passed to CGPatternCreate. (#6356)

We may get callbacks after the managed CGPattern instance has been
disposed/garbage collected, so make sure our delegates survives that long.

Since the delegates don't need any instance state, just make them static.

src/CoreGraphics/CGPattern.cs

@@ -79,7 +79,11 @@ namespace CoreGraphics {
 			/* CGFloat */ nfloat xStep, /* CGFloat */ nfloat yStep, CGPatternTiling tiling, bool isColored,
 			/* const CGPatternCallbacks* */ ref CGPatternCallbacks callbacks);
 
-		CGPatternCallbacks callbacks;
+		static CGPatternCallbacks callbacks = new CGPatternCallbacks () {
+			version = 0,
+			draw = DrawCallback,
+			release = ReleaseCallback,
+		};
 		GCHandle gch;
 		
 		public CGPattern (CGRect bounds, CGAffineTransform matrix, nfloat xStep, nfloat yStep, CGPatternTiling tiling, bool isColored, DrawPattern drawPattern)
@@ -87,10 +91,6 @@ namespace CoreGraphics {
 			if (drawPattern == null)
 				throw new ArgumentNullException (nameof (drawPattern));
 
-			callbacks.draw = DrawCallback;
-			callbacks.release = ReleaseCallback;
-			callbacks.version = 0;
-
 			gch = GCHandle.Alloc (drawPattern);
 			Handle = CGPatternCreate (GCHandle.ToIntPtr (gch), bounds, matrix, xStep, yStep, tiling, isColored, ref callbacks);
 		}