[iOS] Make sure spawnee's isGpuDisabled is set correctly when FlutterEngine spawn (#29361)

Author: ColdPaleLight

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

@@ -1034,6 +1034,7 @@ - (FlutterEngine*)spawnWithEntrypoint:(/*nullable*/ NSString*)entrypoint
   result->_threadHost = _threadHost;
   result->_profiler = _profiler;
   result->_profiler_metrics = _profiler_metrics;
+  result->_isGpuDisabled = _isGpuDisabled;
   [result setupShell:std::move(shell) withObservatoryPublication:NO];
   return result;
 }

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

@@ -24,10 +24,12 @@ - (void)testMake {
 - (void)testSpawn {
   FlutterEngineGroup* group = [[FlutterEngineGroup alloc] initWithName:@"foo" project:nil];
   FlutterEngine* spawner = [group makeEngineWithEntrypoint:nil libraryURI:nil];
+  spawner.isGpuDisabled = YES;
   FlutterEngine* spawnee = [group makeEngineWithEntrypoint:nil libraryURI:nil];
   XCTAssertNotNil(spawner);
   XCTAssertNotNil(spawnee);
   XCTAssertEqual(&spawner.threadHost, &spawnee.threadHost);
+  XCTAssertEqual(spawner.isGpuDisabled, spawnee.isGpuDisabled);
 }
 
 - (void)testDeleteLastEngine {