Blender Git Loki

Workbench: Depth Of Field: Optimisation

- Compute samples positions on CPU.
- Use 3x3 Box blur instead of 2x2.
- Implement bokeh parameters.

With this commit, dof performance is almost negligeable.
The quality is a bit lower than before but can be improve. Also now big
Circle of confusion are supported (up to 200px).
Cost is ~1.25ms on AMD Vega with a 2560p viewport than full HD and
pretty shallow depth of field.

Coc downsampling and dilation is not used anymore for now (commented).