Support for ExternalVR implementation
This PR adds the hook necessary for the ExternalVR rust-webvr driver.
Waiting on rust-webvr 0.9.3 to be published before landing.
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shutdown synchronously
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Currently on Android we treat all touch events as scroll events. Servo
is already capable of distinguishing between scroll-touches and regular
touch events (see `on_touch_move` in `components/compositor/touch.rs`),
so we should just be passing touch events through.
Servo however does not natively support fling gestures, so we continue
to use `GestureDetector` for that.
Properly cancel looping before triggering another drawing loop
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- [x] These changes fix#21991 (github issue number if applicable).
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