Replace deprecated [UIScreen mainScreen] in FlutterView.mm (#46802)

Addresses: flutter/flutter#130828

Author: mossmana

shell/platform/darwin/ios/framework/Source/FlutterView.h

@@ -46,6 +46,8 @@
                           opaque:(BOOL)opaque
                  enableWideGamut:(BOOL)isWideGamutEnabled NS_DESIGNATED_INITIALIZER;
 
+- (UIScreen*)screen;
+
 // Set by FlutterEngine or FlutterViewController to override software rendering.
 @property(class, nonatomic) BOOL forceSoftwareRendering;
 @end

shell/platform/darwin/ios/framework/Source/FlutterView.mm

@@ -100,7 +100,7 @@ - (void)layoutSubviews {
 #pragma clang diagnostic ignored "-Wunguarded-availability-new"
     CAMetalLayer* layer = (CAMetalLayer*)self.layer;
 #pragma clang diagnostic pop
-    CGFloat screenScale = [UIScreen mainScreen].scale;
+    CGFloat screenScale = self.screen.scale;
     layer.allowsGroupOpacity = YES;
     layer.contentsScale = screenScale;
     layer.rasterizationScale = screenScale;

shell/platform/darwin/ios/framework/Source/FlutterViewTest.mm

@@ -69,4 +69,17 @@ - (void)testIgnoreWideColorWithoutImpeller {
   XCTAssertEqual(layer.pixelFormat, MTLPixelFormatBGRA8Unorm);
 }
 
+- (void)testLayerScalesMatchScreenAfterLayoutSubviews {
+  FakeDelegate* delegate = [[[FakeDelegate alloc] init] autorelease];
+  FlutterView* view = [[FlutterView alloc] initWithDelegate:delegate opaque:NO enableWideGamut:NO];
+  view.layer.contentsScale = CGFloat(-99.0);
+  view.layer.rasterizationScale = CGFloat(-99.0);
+  UIScreen* screen = [view screen];
+  XCTAssertNotEqual(view.layer.contentsScale, screen.scale);
+  XCTAssertNotEqual(view.layer.rasterizationScale, screen.scale);
+  [view layoutSubviews];
+  XCTAssertEqual(view.layer.contentsScale, screen.scale);
+  XCTAssertEqual(view.layer.rasterizationScale, screen.scale);
+}
+
 @end