SL-18095 WIP -- Add Mikktspace tangent generation for PBR materials and switch to per-pixel binormal generation.  Still bugged with some test content.

Nicky Dasmijn's contribution for zlib-ng Linux builds

On Mac, execute the packaged SLVersionChecker directly rather than invoking
system Python on our manually-bundled scripts.

This changeset makes it possible to build the Second Life viewer using
Python 3. It is designed to be used with an equivalent Autobuild branch
so that a developer can compile without needing Python 2 on their
machine.

Breaking change: Python 2 support ending

Rather than supporting two versions of Python, including one that was
discontinued at the beginning of the year, this branch focuses on
pouring future effort into Python 3 only. As a result, scripts do not
need to be backwards compatible. This means that build environments,
be they on personal computers and on build agents, need to have a
compatible interpreter.

Notes

- SLVersionChecker will still use Python 2 on macOS
- Fixed the message template url used by template_verifier.py

This reverts commit 0a745b47.