Blender Git Loki

Cycles: Tweaks to the resolution divider

- Base initial resolution divider on a pixel size.

Using hardcoded low resolution provides rather annoying blobby
viewport on first enter of rendered viewport. Set the divider in
this case to 8 of the native pixel sizes. A bit of heuristic, which
seems to give more appealing pixel size, and kind of matches what
is possible to setup as fastest preview with the pixel size setting.

- Fix miscalculation of how long viewport number of samples will take.
Need to properly take number of samples during update into account.
Seems to make Junk Shop more interactive on home PC. Although, it
was already feeling much better than master.

- Always render 4 samples for highest resolution divider.
Before it was clamped to the number of samples. There is an
explanation in the code but the idea is that this is not slower and
does bring extra information during navigation.