Implement DeepCAttributeTransfer node

[charlesangus]

Like a Deep equivalent of Shuffle Copy, but since Deep deals in depth samples as well as pixels, it should probably be a bit more like Houdini's AttributeTransfer node.

Modes I see needing:

• 2d input for attribute source, distribute new value along samples, basically like a DeepRecolor but handing just a secondary channel (although now I'm thinking of it, Deep Recolor may already be fine for this...).
• Deep input for attribute source, with tolerance settings for transferring attributes, such as:
  • World radius, based on PWorld attribute
  • Z tolerance, so forward or backward in the same pixel
  • Image-based radius, so within a certain number of pixels in X and Y and likely Z
  • probably also kernels for interpolation, like linear, Gaussian, etc (see what Houdini offers)

[charlesangus]

Use case is e.g. rendering with a system that doesn't support multiple AOVs in the same file, so needing to combine multiple Deep renders into one stream. This can kind of be done with DeepMerge to an extent, but it's not quite right, and also merges other channels and samples if present, which may not be desirable.

I can also see a use case in transferring attributes from one render to another when merging, to e.g. further process their intersection points later on, possibly with a DeepCGrade or similar, or after flattening.

So should be able to pick source channels and destination channels.