Bug 1610952 - Ensure webrender's async scroll offsets are in correct coordinate space. r=botond

For zoomable APZCs, we were correctly dividing the scroll offset by
the pinch zoom scale. This is effectively equal to multiplying by the
device pixel ratio then dividing by the zoom, converting the offset
from parent layer space to layout device space.

However, for non-zoomable APZCs, we were incorrectly assuming that the
pinch zoom scale equalled 1.0. This was causing us to send incorrectly
scaled async scroll offsets to webrender, resulting in content moving
too slowly or quickly whilst asynchronously panning, then suddenly
jumping as the synchronous scroll offset caught up.

This is fixed by ensuring we always divide the asynchronous scroll
offset by the pinch zoom scale, regardless of whether the APZC is
zoomable or not.

Differential Revision: https://phabricator.services.mozilla.com/D61315

--HG--
extra : moz-landing-system : lando

gfx/layers/apz/src/APZCTreeManager.cpp

@@ -711,12 +711,12 @@ void APZCTreeManager::SampleForWebRender(wr::TransactionWrapper& aTxn,
         apzc->GetCurrentAsyncTransform(AsyncPanZoomController::eForCompositing,
                                        asyncTransformComponents)
             .mTranslation;
-    LayoutDeviceToParentLayerScale zoom;
+    LayoutDeviceToParentLayerScale zoom =
+        apzc->GetCurrentPinchZoomScale(AsyncPanZoomController::eForCompositing);
+
     if (Maybe<uint64_t> zoomAnimationId = apzc->GetZoomAnimationId()) {
       // for now we only support zooming on root content APZCs
       MOZ_ASSERT(apzc->IsRootContent());
-      zoom = apzc->GetCurrentPinchZoomScale(
-          AsyncPanZoomController::eForCompositing);
 
       AsyncTransform asyncVisualTransform =
           apzc->GetCurrentAsyncTransform(AsyncPanZoomController::eForCompositing,