Blender Git Loki

Cycles: Schedule more samples to path trace work

The new scheduling goes as following:

- Render 1 sample, give visual feedback, and see how long it takes to
render a sample.

- After that schedule as many samples as needed to not have more frequent
updates then once a second.

Currently there are no measurable difference, but it will become more
visible once we'll be scheduling more than one sample to GPU.