Blender Git Loki

Render: Only run callbacks for non-preview renders

Those handlers are usually done to edit scene which is being rendered,
and this is not supported due to a fully localized nature of the preview
bmain.

There is still non-conditional callback in stats_background which is a
bit tricky to cover with check, but this code is not supposed to be run
for previews anyway.

Reviewers: brecht

Differential Revision: https://developer.blender.org/D5731