extract_unity_shaders: Refactor shader asm handler into separate function

Going to generate a new node when adding fog to a shader, so move this
into a separate function.
extract_unity_shaders: Pass args down the callchain
extract_unity_shaders: Add --fog to generate fog variants

This will be useful to:

a) Match headers for a shader that has had the fog instructions added
b) Autofix all possible vertex shaders in a game without requiring dumps
   from Helix mod