extract_unity_shaders: Detect and warn about CRC32 collisions

This will detect CRC32 collisions on distinct shaders - i.e. where two
or more *different* shaders have the same checksum, making using them in
Helix mod dangerous as they will override a completely different effect.

This may be unlikely, but I'd rather know about it before I get bitten.

Of course, this can only detect collisions if it is parsing the
colliding shaders simultaneously.