v0.8.1 patchlevel 2

Changes since pl1:

• f1tattoo: fixed access mode for mingw (@david4599)
• f1tattoo: fixed png feature faulty return values and preprocessor statement (@david4599)
• updated tests and build workflows

assets:

• AppImage: self-contained portable Linux binary, works on all distros
• win32-release.zip: portable Windows XP+ binary (32 bits, take this one if you're unsure)
• win64-release.zip: portable Windows XP+ binary (64 bits)