This was not properly respected, and in general with multiple passes and layers
it's unclear what this should do exactly without breaking some render passes.
Better to keep this image format for raw unmodified render results.
This function did multiple things making it difficult to know
what was intended by the caller:
- Directory & file join.
- Expand relative '//' prefix to an optional directory.
- Expand drive letters on windows (guessing with fall-backs).
- Switch slashes to native direction.
This functionality wasn't needed as the full directory was always passed
in, so guessing the drive letter wasn't needed.
If functionality to add drive letters onto paths is needed in the future
a function that only does this can be added.
