overlay_stack: Always propagate button-release event to selected overlay.

Authored by stefanzzz on Aug 1 2017, 2:16 PM.

Description

overlay_stack: Always propagate button-release event to selected overlay.

Make sure the button-release event is propagated to the selected
overlay even if the release happened outside of the viewer. Otherwise,
the started UndoableAction is not commited.

Fixes T7800.

Differential Revision: https://phabricator.freedesktop.org/D1817