Blender Git Loki

Cycles: Further improvements to the viewport rendering

The idea is to schedule more samples at a lower resolution, which
gives more usable visual feedback. For example, The Junk Shop scene
becomes a bit less resolution but is way less noisy (almost no black
pixels) while steel feeling interactive.

Currently schedule 4 samples during navigation, 2 samples for the
resolution which is the previous to the final one. When denoising is
enabled during navigation, schedule single sample (which makes it so
higher resolution will be rendered). This is because both denoisers
we have behave better on more noisy image but on higher resolution.

Additionally, lower the update rate in viewport. Non-trivial scenes
can not keep up 60fps without dropping resolution too much. So prefer
less often updates but have more clean updates during navigation.