Torch doesn't support TLUTs that are in a different file and share offsets

OOT support added the key ` external_tlut` and `external_tlut_offset` but neither seem to be used anywhere.
Ship doesn't technically need these since it never needs to combine files that use these into a normal rgba image but it is still a feature ZAPD had that Torch doesn't yet.

Search the Torch source for the external_tlut and external_tlut_offset keys and trace how TLUTs are loaded and combined into RGBA images. Compare the handling of same-file TLUTs with external TLUTs that share offsets; done means those external definitions are honored without breaking existing asset processing.

Written by the indexing model from the issue text.