Blender Git Loki

Cycles: Fix possible stalling in viewport updates

If rendering a sample takes longer than update timeout it was
possible to have viewport navigation which will never update a
viewport during navigation.

For example, rendering on CPU with high start resolution and
denoising enabled could have taken a bit more than configured
0.1sec. Before this change the viewport would have only updated
after navigation was finished. After this change the viewport
will be refreshed at about 10fps, which is better for artists.