Blender Git Loki

Merge per-shader SVM nodes in the main update thread to avoid locking and resizing overhead

In a test file containing 1300 copies of the same shader, this reduces shader update time from 3.1 sec to 0.05 sec.

Thanks to @swerner for noticing this issue.

Reviewers: brecht, sergey, swerner

Subscribers: swerner

Differential Revision: https://developer.blender.org/D5376