Blender Git Loki

Fix pass accessor with 0 samples

Avoid division by zero when calculating scale, and avoid multiplication
by inf later on.

The pixel processing functions are changed so that they return fully
transparent pixel for pixels where number of samples is 0. This allows
to have matched behavior of non-finished tiles with master, without
relying on non-finite math.

The extra condition when calculating scale is inevitable, and hopefully
the pixel processing function's if statement gets folded into it as
well ()due to force-inline).

Differential Revision: https://developer.blender.org/D12518