extract_unity_shaders: combine headers of duplicate shaders WIP
export_unity_shaders: Detect and handle duplicate shaders & variants

- Duplicate variants of the same shader will only be written once. The
  filename will be a combination of the union of all keywords, passes
  and subshaders. Headers will be merged with differing lines indicated.

- Duplicate variants of different shaders will still be saved in each
  shader directory. The headers for each will be placed one after
  another with the top line indicating which shaders were matched.

- Combinations of duplicate variants within a single shader and across
  several shaders are supported simultaneously. Headers of the same
  shader are merged, and of different shaders are appended. The first
  line of the headers indicates how many variants and shaders were
  matched. Each distinct shader will have one combined output file.