TOOLS: Restore support for DXA compression type 12

[ccawley2011]

Note that compared to the original 2006 version of the code, this PR explicitly specifies the endianness of the map word - previously it was written to the file (and read in by ScummVM itself) in a way that isn't endian-safe. Since there doesn't appear to be any release versions of ScummVM Tools that support type 12 and I couldn't find any example files in the wild, this shouldn't be a major issue.

This includes the changes from PR #67.