Change smooth zoom threshold from 200% to 400%

Summary:
At zoom levels >= 200%, scaling changed from smooth to fast, i.e.
actual pixels started showing.
This patch changes it to 400%.

BUG: 270980

Test Plan:
Open different sized raster images.
Zoom to at least 400%.
Image should only look pixellated at 400% and higher zooms.

Reviewers: #gwenview, rkflx

Reviewed By: #gwenview, rkflx

Subscribers: ngraham, rkflx

Tags: #gwenview

Differential Revision: https://phabricator.kde.org/D12483