Use plain color source instead of gradient in KisPresetLivePreviewView
Summary:
KisPresetLivePreviewView cannot display gradient color source: there is
no resource manager for KisResourcesSnapshot, therefore gradient is
nullptr. That crashes Krita, when it tries to render the preview.
This patch solves the issue by falling back to plain color source for
preset preview rendering.
BUG:385521
Reviewers: Krita, rempt
Reviewed By: Krita, rempt
Tags: Krita
Differential Revision: https://phabricator.kde.org/D19297