Blender Git Loki

Cycles X: Move top-level pass access log to a base class

What this change allows to do is to share top-level logic of pixel
access between various implementations (CPU, GPU), while keeping the
actual pixel processing very small.

Currently there are no functional changes, as well no measurable
difference in timing spent in getting tile pixels.

Differential Revision: https://developer.blender.org/D11477