Fix image smoothing sometimes not getting applied

Summary:
In {nav View} mode, smoothing is applied for scaling up raster images up
to 400%. In general this works fine, but in some cases the smoothing
becomes visible only after zooming manually.

This is due to defaulting to `Qt::FastTransformation` and switching to
`Qt::SmoothTransformation` only for changes in the zoom level and for
the {nav Fit/Fill} zoom modes, but not when the zoom level is unchanged
when navigating to an image.

The issue can be fixed by calling `onZoomChanged`, which not only calls
`updateBuffer` as before, but also `setTransformationMode`.

BUG: 396736
FIXED-IN: 18.08.0

Test Plan:
Enable {nav Image View > Zoom mode > Keep same zoom and position}, zoom
in to a zoom level just below 400% and switch to the next image.
Smoothing should be applied immediately, without having to zoom in and
out again.

Reviewers: #gwenview, ngraham

Reviewed By: #gwenview, ngraham

Subscribers: muhlenpfordt, ngraham

Differential Revision: https://phabricator.kde.org/D14269