Avoid unnecessary allocation of temporary buffers for each mip, and creates
only one Image with the compressed data.
Also renames variable and reorders code for clarity.
Clarify that squish is now only used for decompression.
Documented which formats can be decompressed in Image.
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>
