v1.0: major rewrite

Main changes:

• webp2png is now merged into png2webp, accessible through the -r flag.
• -b flag removed as this is now the default, no more guesswork.
• -v prints basic info about PNGs.
• Dropped netpbm support.
• Better error handling:
  • Deletes output files on error.
  • Continues iterating through files in batch mode.
• Windows release builds are now through MSYS2 Clang64, as the builds I put out before were using an unpublished hacky script to get MSVC to optimise well, and therefore the builds weren't reproducible.

Attached png2webp.exe compiled with MSYS2 CLANG64 x86_64-w64-mingw32-clang 14.0.4-2(?).
SHA256: a3aaba96e7f02d0442b1909933f8321d1e125e09213b1f34ed07f90043af3f02