This add a new set of (possible) render settings that can be defined at
the scene level and overridable at the scene layer level.
Once we get workspaces we can either add workspace inbetween scene and
scene layer evaluation. Or to replace layer settings, to avoid extra
confusion to users.
An example of this setting is "samples", as implemented now for the clay
engine.
We can now use object and other modes on top of Cycles.
Since we are now always on "render_to_view" (old Rendered mode), the
pause button is always visible.
