Handle cancelled touches on iOS

[tryk016]

UIKit may cancel an active touch when the app loses the foreground or a system gesture interrupts input. The iOS backend handles began, moved and ended touches, but currently ignores cancellation, leaving the touch registered and the corresponding mouse button pressed.

Forward touchesCancelled through the existing touchesEnded path so Tempest removes the touch ID and emits the matching mouse-up event.

Validation:

• Release builds pass for iPhoneOS arm64 and iPhone Simulator arm64 at iOS 15.0
• Clang static analysis reports no new issue
• on an iPhone 15 Pro Max, backgrounding OpenGothic while holding movement releases the input and subsequent touches continue to work