[CP][iOS] always post new task during gesture dispatch. (flutter#163666)

CP for flutter#163646

Fixes flutter#163429

Author: Jonah Williams

CHANGELOG.md

@@ -97,6 +97,7 @@ Initial stable release.
 
 - [flutter/163304](https://github.com/flutter/flutter/issues/163304) Fixes crash when using backdrop filter on GLES backend.
 - [flutter/161334](https://github.com/flutter/flutter/issues/161334) Disable Vulkan on certain Xclipse GPU models.
+- [flutter/163429](https://github.com/flutter/flutter/issues/163429) Fixes jittery scrolling on iOS devices.
 
 ## Flutter 3.27 Changes