- `etc` module was renamed to `etcpak` and modified to use the new library.
- PKM importer is removed in the process, it's obsolete.
- Old library `etc2comp` is removed.
- S3TC compression no longer done via `squish` (but decompression still is).
- Slight modifications to etcpak sources for MinGW compatibility,
to fix LLVM `-Wc++11-narrowing` errors, and to allow using vendored or
system libpng.
Co-authored-by: Rémi Verschelde <rverschelde@gmail.com>
