Populate imageClickedPoint when submitter is an image button.
If the form submission originated from an event dispatch, this patch
ensures that the submitted image button coordinate is not the default.
Differential Revision: https://phabricator.services.mozilla.com/D224651
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=282266
gecko-commit: e37003ee9eec69cb07182f1656326725961bc183
gecko-reviewers: dom-core, layout-reviewers, emilio, masayuki