[codex] Add release AppImage CI

[vitaly-zdanevich]

Summary

Adds automated AppImage packaging for Flowblade releases, with a reusable local build script and CI validation path.

The workflow builds an x86_64 AppImage on release events, uploads it as a workflow artifact, and attaches it to the GitHub release when the event is an actual release. It can also be run on the validation branch used for this PR.

Prepared with Codex (GPT-5, xhigh reasoning).

Runtime Packaging

• Bundles Flowblade runtime dependencies, Python GI typelibs, GTK/GDK resources, MLT plugins/data, fonts, translations, pixbuf loaders, and supporting shared libraries.
• Configures the AppImage launcher for X11 GTK/SDL backends by default, which avoids missing Wayland library failures on non-Wayland hosts.
• Sets AppImage-local fontconfig and GDK pixbuf paths.
• Includes fixes for missing runtime pieces found during manual and CI smoke testing, including typelibs such as xlib/HarfBuzz and libraries such as libwayland-client and libcom_err.

Smoke Tests

• Adds a direct AppImage startup smoke test.
• Adds container smoke coverage for Ubuntu 22.04, Ubuntu 24.04, Debian 12, Debian 13, Fedora, Arch Linux, and openSUSE Tumbleweed.
• Leaves Gentoo as an optional smoke image because the currently tested Gentoo binary package path did not provide the required Xvfb package reliably enough for default CI.
• Uses dummy SDL audio during CI smoke tests so headless runners without an audio device do not fail after Flowblade reaches playback initialization.

Flowblade Runtime Fix

Fixes the MLT SDL consumer selection for the bundled Ubuntu 22.04 MLT 7.4 runtime. The previous version comparison used string ordering, so 7.4.0 was treated as newer than 7.28.0 and Flowblade selected the SDL2 path. That caused the separate black MLT window during AppImage playback tests.

This PR restores the SDL1 path for MLT versions older than 7.28.0 and keeps the SDL2 path available for newer MLT versions or explicit force mode.

Validation

• bash -n packaging/appimage/build-appimage.sh packaging/appimage/smoke-test-appimage.sh packaging/appimage/smoke-test-appimage-containers.sh
• python3 -m py_compile flowblade-trunk/Flowblade/src/app.py flowblade-trunk/Flowblade/src/editorstate.py flowblade-trunk/Flowblade/src/tools/scripttool.py flowblade-trunk/Flowblade/src/tools/gmic.py
• Local rebuilt AppImage smoke passed with SDL1 consumer startup.
• Manual Gentoo/X11 AppImage run confirmed by tester after the SDL consumer fix.
• Fork CI passed on the squashed commit: https://github.com/vitaly-zdanevich/flowblade/actions/runs/27852951649

[vitaly-zdanevich]

@Samueru-sama
@ivan-hc

Feel free to review.

[ivan-hc]

@Samueru-sama @ivan-hc

Feel free to review.

Your coding is far from my understanding, I use other methods to package AppImages. I can't help much.