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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Android's `View` has multiple constructors. Currently only `ServoView(Context, AttributeSet)` is available.
This constructor is usually used by Android when inflating a View from a layout XML file. However when
creating the `View` in code no `AttributeSet` is available.
Make JNI code more robust and introduce antialiasing option
This throws exceptions in various places. Trying to reduce the amount of unwrap/expect.
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Fix flickering on Android
This is particularly noticeable in debug builds on WebGL pages, but also appears during startup when loading normal pages. requestRender() causes Android to swap buffers under the assumption that the buffer contains a fully rendered frame, but when calling it from `doFrame` there are no guarantees that Servo has finished compositing yet. This causes stale buffers to be composited instead, leading to flickering at startup when there is no content, or general jerkiness on pages using animation callbacks as previous frames replace current frames.
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