Fixed canvas isn't cleared when adding a new blank frame
Summary:
When create blank frame is clicked on animation docker having a keyframe selected, that is not the last frame in the timeline, the canvas is not cleared properly.
Note from Dmitry Kazakov: setDirty() should be called after the
actual device change has been done. Otherwise race condition may happen.
BUG: 403535
FIXED-IN: krita 4.2.0-pre-alpha (git 0d8b6bd)
Reviewers: Krita, dkazakov!
Subscribers: dkazakov
Tags: Krita
Differential Revision: https://phabricator.kde.org/D18790