Find a way to support multiple render passes or merge OpenEXR Multi-Layer files

[BeheadedKamikaze]

Ideas:

1. Support OpenEXR Multi-Layer files
   This would be the ideal solution.
   Edit: OK, so I have read the Blender docs, C source code, and developer website. This is currently not possible. 😞
   There are multiple issues open about it since Blender 2.6x and little progress has been made so far.

2. Save out each render pass to its own separate set of files
   This will at least allow us to implement a solution, even if it's not ideal.
   As we're rendering each tile, we can use a bunch of File Output nodes to save each render pass into its own (separate) EXR file, perhaps with a filename prefix to identify the render pass, or maybe they can just go into different directories for each render pass.
   Once rendering is complete, then each set can be combined individually, and saved out to a target EXR.

Care will need to be taken to combine passes with the correct image types; i.e. there are passes with vector data (e.g. RGB, RGBA, XYZ, UVW), and there are passes with float data (Alpha, Depth). We will also need to know this to create the File Output node, to set the correct Colour format (BW, RGB, RGBA).

Also need to consider how we might be able to save outputs from other compositor nodes (like SID)... 🤔