Pull out CGContext early in UIImage+Diff (#35940)

Summary:
This is a small refactor designed to make future merges for React Native macOS easier. It was found while merging React Native 0.71 into React Native macOS.

In React Native macOS, we ifdef iOS  out API's and replace them with macOS APIs, reusing as much code as possible. `CGContext` is a type that is available on both iOS and macOS, but `UIGraphicsImageRendererContext` is not. A simple refactor makes it easier to reuse this code in React Native macOS without affecting iOS :)

Resolves microsoft#1676

## Changelog

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[IOS] [CHANGED] - Pull out CGContext early in UIImage+Diff

Pull Request resolved: #35940

Test Plan: Build should pass.

Reviewed By: matrush

Differential Revision: D42761424

Pulled By: javache

fbshipit-source-id: 5ce79a5e7c60484dd37d0b2c3f58ff0897d662df

packages/rn-tester/RCTTest/FBSnapshotTestCase/UIImage+Diff.m

@@ -21,15 +21,16 @@ - (UIImage *)diffWithImage:(UIImage *)image
   UIGraphicsImageRenderer *const renderer = [[UIGraphicsImageRenderer alloc] initWithSize:imageSize
                                                                                    format:rendererFormat];
 
-  return [renderer imageWithActions:^(UIGraphicsImageRendererContext *_Nonnull context) {
+  return [renderer imageWithActions:^(UIGraphicsImageRendererContext *_Nonnull rendererContext) {
+    const CGContextRef context = rendererContext.CGContext;
     [self drawInRect:CGRectMake(0, 0, self.size.width, self.size.height)];
-    CGContextSetAlpha(context.CGContext, 0.5f);
-    CGContextBeginTransparencyLayer(context.CGContext, NULL);
+    CGContextSetAlpha(context, 0.5f);
+    CGContextBeginTransparencyLayer(context, NULL);
     [image drawInRect:CGRectMake(0, 0, image.size.width, image.size.height)];
-    CGContextSetBlendMode(context.CGContext, kCGBlendModeDifference);
-    CGContextSetFillColorWithColor(context.CGContext, [UIColor whiteColor].CGColor);
-    CGContextFillRect(context.CGContext, CGRectMake(0, 0, self.size.width, self.size.height));
-    CGContextEndTransparencyLayer(context.CGContext);
+    CGContextSetBlendMode(context, kCGBlendModeDifference);
+    CGContextSetFillColorWithColor(context, [UIColor whiteColor].CGColor);
+    CGContextFillRect(context, CGRectMake(0, 0, self.size.width, self.size.height));
+    CGContextEndTransparencyLayer(context);
   }];
 }