exo: Dispatch input events to root surface for ARC

Client-driven dragging/resizing relies on implicit grab. However, the
client may destroy the focused surface, which prematurely ends the
drag/resize. This CL prevents this by enforcing that the target for
input events is always the root surface, which has the same lifetime
as the client-side window.

Bug: b:72627652
Test: Wine applications can be resized by dragging window borders.
Change-Id: I9badb6624ec32bd1e1286dc19b7ace0a8b136ff5
Reviewed-on: https://chromium-review.googlesource.com/911812
Commit-Queue: Dominik Laskowski <[email protected]>
Reviewed-by: David Reveman <[email protected]>
Cr-Commit-Position: refs/heads/master@{#535781}
11 files changed