This started as a fixing a misalignment issue, but I got carried away.
Was originally 41 bytes (overflowing it's expected 40 bytes by 1 bit).

Now it's 27 bytes plus 8 bits of padding (ready for future expansion).

The savings come from:
 - Removing UV usage from components, it can be reconstructed from
   texcoord_elem_count (saved 8 bits)
 - Removed the 8 unused bits from texMtxInfo_n_projection (saved 8 bits)
 - Removing unused bit from from start of components (saved 1 bit)
 - Removed extra bit from inputform, texgentype and sourcerow
 - packed texMtxInfo_n_projection and texcoord_elem_count back into
   the per texgen info space freed up above. (saved 24 bits)
 - Overlayed postMtx index and Emboss mode shifts into a union based on
   texgentype (saved 56 bits)
 - Move to a single-bit union tag, freeing up an extra bit for regular
   texgens.
 - Move PotMtx normalize into the freed up space (saved 8 bits)

Total savings: 105 bits of data

EDIT: Even worse, MSVC wasn't respecting pack(1) for bitfields at all,
      so on windows this struct was actually quite a bit larger.
      Something like 80 bytes, if not more.

      Fixed this by dropping the size of any enums used in these
      bit structs to u8. Our Common::BitField allows for the underlying
      type to be explicitly specified to something larger.

      msvc's bitfield packing appears to be completely braindead, can't
      mix sizes at all.

We can just tag the std:: onto the end of the specialization to make it
less noisy.

Also mark it as noexcept, since hashes shouldn't throw exceptions.

Fixes an issue where the default initializer wouldn't initialize padding, making for fun non-determinism

SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.