Make EventStateManager try to dispatch pointer boundary events before dispatching an `ePointerUp
"fire a pointer event" is defined as:
https://w3c.github.io/pointerevents/#dfn-fire-a-pointer-event
Before firing this event, the user agent SHOULD treat the target as if the
pointing device has moved over it from the previousTarget for the purpose of
ensuring event ordering.
Therefore, pointerup should imply a pointer move and cause pointer boundary
events before it's dispatched (and after that if non-hoverable pointer, see
pointerup definition about this). In most cases, it's not required when the
input device does not support hover state. However, if the touch target is
removed from the tree, the removed element stopped capturing the pointer. In
this case, we need to synthesize pointer boundary events before dispatching
pointerup like inputs from hoverable pointer.
Differential Revision: https://phabricator.services.mozilla.com/D206209
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1888013
gecko-commit: 9317687169b17887ad155591bfa2049a70fe418d
gecko-reviewers: smaug, edgar, dom-core