We do our own image loading, threading, and memory management in Godot already,
so the only components we need from etcpak (at least as of now) are the
`Compress*` methods defined in `ProcessDxtc.cpp` and `ProcessRGB.cpp`.
So we don't need to compile or vendor the rest.
- `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>
