New Layer from visible
Summary:
its a common function under Layer->New menu which is in gimp and ps.
It create /insert layer on top from visible pixels
Test Plan: u can create several layers with some painting and then test this function which create new layer with all paintings together
Reviewers: rempt, dkazakov
Reviewed By: dkazakov
Differential Revision: https://phabricator.kde.org/D2428