Currently resolution divider is not exposed to the
interface yet, and i'm not even sure it needs to be
exposed because it's somewhat weird configuration.
Need to check how often artists are changing start
resolution in Cycles.
Pretty much straightforward implementation with the
only weak part: render result is getting re-allocated
and upscaled when current resolution is finished.
Not sure how to make it faster actually. Maybe it's
just a matter of making upscale fast enough.
Needed to fix some possible memory leak happening
in Freestyle when canceling rendering on a special
stage -- it was missing temp bmain free,
Reviewers: campbellbarton, dingto
CC: sebastian_k, fsiddi, venomgfx
Differential Revision: https://developer.blender.org/D609
ca0c0e7549