Blender Git Loki

Cycles: Update remaining time once per second without waiting for a tile change

Previously, the code would only update the status string if the main status changed.
However, the main status did not include the remaining time, and therefore it wasn't updated until the amount of rendered tiles (which is part of the main status) changed.

This commit therefore makes the BlenderSession remember the time of the last status update and forces a status update if the last one was more than a second ago.

Reviewers: sergey

Differential Revision: https://developer.blender.org/D2465