build: add AppImage packaging script

packaging/appimage/AppRun

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+#!/bin/sh
+# Start the real binary by its path inside the AppImage. A symlinked AppRun makes Qt take
+# the AppImage root as the application dir, so it misses usr/bin/qt.conf and finds no
+# platform plugins.
+HERE="$(dirname "$(readlink -f "$0")")"
+# The Plasma platform theme's own file dialog lists folders through KIO workers, which are
+# not bundled (it would show every folder as empty). Use the host's dialog via the
+# xdg-desktop-portal instead.
+export PLASMA_INTEGRATION_USE_PORTAL=1
+export XDG_DATA_DIRS="$HERE/usr/share:${XDG_DATA_DIRS:-/usr/local/share:/usr/share}"
+exec "$HERE/usr/bin/qtpotato" "$@"

packaging/appimage/build-appimage.sh

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+#!/usr/bin/env bash
+# Build a QtPotato AppImage from the host's Qt/KF6/mpv libraries.
+#
+# The result only runs on distros whose glibc is at least as new as the build host's,
+# so build on the oldest distro you want to support.
+#
+# Usage: packaging/appimage/build-appimage.sh [build-dir]
+set -euo pipefail
+
+SRC_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")/../.." && pwd)"
+BUILD_DIR="$(realpath -m "${1:-$SRC_DIR/build-appimage}")"
+TOOLS_DIR="${TOOLS_DIR:-${XDG_CACHE_HOME:-$HOME/.cache}/qtpotato-appimage-tools}"
+APPDIR="$BUILD_DIR/AppDir"
+QT_PLUGINS="$(qmake6 -query QT_INSTALL_PLUGINS)"
+
+export APPIMAGE_EXTRACT_AND_RUN=1 # works without FUSE
+# The strip bundled with linuxdeploy is too old for binaries from current distros
+# (e.g. DT_RELR sections) and aborts the deployment.
+export NO_STRIP=1
+
+fetch_tool() {
+    local repo=$1 file=$2
+    if [[ ! -x "$TOOLS_DIR/$file" ]]; then
+        curl -fL --create-dirs -o "$TOOLS_DIR/$file" \
+            "https://github.com/linuxdeploy/$repo/releases/download/continuous/$file"
+        chmod +x "$TOOLS_DIR/$file"
+    fi
+}
+fetch_tool linuxdeploy linuxdeploy-x86_64.AppImage
+fetch_tool linuxdeploy-plugin-qt linuxdeploy-plugin-qt-x86_64.AppImage
+fetch_tool linuxdeploy-plugin-appimage linuxdeploy-plugin-appimage-x86_64.AppImage
+export PATH="$TOOLS_DIR:$PATH"
+
+cmake -S "$SRC_DIR" -B "$BUILD_DIR" -G Ninja \
+    -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=/usr -DBUILD_TESTING=OFF
+cmake --build "$BUILD_DIR"
+rm -rf "$APPDIR"
+DESTDIR="$APPDIR" cmake --install "$BUILD_DIR"
+
+# Plugins that are loaded at runtime by name, so linuxdeploy-plugin-qt cannot find them:
+# Kirigami's style plugins, the Plasma platform theme (colors, fonts, icon theme and the
+# QtQuick style from the user's Plasma settings) and the Breeze/Union widget styles.
+copy_plugins() {
+    local subdir=$1; shift
+    mkdir -p "$APPDIR/usr/plugins/$subdir"
+    for pattern in "$@"; do
+        for f in "$QT_PLUGINS/$subdir"/$pattern; do
+            if [[ -e "$f" ]]; then cp "$f" "$APPDIR/usr/plugins/$subdir/"; fi
+        done
+    done
+}
+copy_plugins kf6/kirigami/platform '*.so'
+copy_plugins platformthemes KDEPlasmaPlatformTheme6.so
+copy_plugins styles breeze6.so UnionWidgetsStyle.so
+copy_plugins union '*.so'
+copy_plugins kf6 FrameworkIntegrationPlugin.so
+# EGL integration for Wayland; without it Qt Quick cannot create an OpenGL context.
+copy_plugins wayland-graphics-integration-client '*.so'
+
+# QtQuick styles and their private modules are also picked at runtime; list them in a
+# dummy QML file so qmlimportscanner deploys them.
+EXTRA_QML="$BUILD_DIR/appimage-extra-qml"
+mkdir -p "$EXTRA_QML"
+cat > "$EXTRA_QML/Styles.qml" <<'EOF'
+import QtQuick
+import QtQuick.Controls
+import QtQuick.Controls.Fusion
+import QtQuick.Templates
+import org.kde.kirigami
+import org.kde.kirigami.platform
+import org.kde.desktop
+import org.kde.qqc2desktopstyle.private
+import org.kde.breeze
+import org.kde.union
+Item {}
+EOF
+
+deps_only=()
+while IFS= read -r -d '' f; do
+    deps_only+=(--deploy-deps-only="$f")
+done < <(find "$APPDIR/usr/plugins" -name '*.so' -print0)
+
+# linuxdeploy-plugin-qt deploys every image format plugin and fails on any with a missing
+# dependency (e.g. kimg_jxr without libjpegxr). Hand it a filtered view of the plugin dir
+# through a qmake wrapper.
+FILTERED_PLUGINS="$BUILD_DIR/appimage-qt-plugins"
+rm -rf "$FILTERED_PLUGINS"
+mkdir -p "$FILTERED_PLUGINS/imageformats"
+for d in "$QT_PLUGINS"/*; do
+    [[ "$(basename "$d")" == imageformats ]] || ln -s "$d" "$FILTERED_PLUGINS/"
+done
+for f in "$QT_PLUGINS"/imageformats/*.so; do
+    if ldd "$f" | grep -q 'not found'; then
+        echo "Skipping $(basename "$f"): missing dependencies"
+    else
+        ln -s "$f" "$FILTERED_PLUGINS/imageformats/"
+    fi
+done
+cat > "$BUILD_DIR/qmake-wrapper" <<EOF
+#!/bin/sh
+if [ "\$1" = -query ] && [ "\$2" = QT_INSTALL_PLUGINS ]; then echo "$FILTERED_PLUGINS"; exit 0; fi
+out=\$(qmake6 "\$@") || exit
+printf '%s\n' "\$out" | sed "s|^QT_INSTALL_PLUGINS:.*|QT_INSTALL_PLUGINS:$FILTERED_PLUGINS|"
+EOF
+chmod +x "$BUILD_DIR/qmake-wrapper"
+
+export QMAKE="$BUILD_DIR/qmake-wrapper"
+export QML_SOURCES_PATHS="$SRC_DIR/src/qml:$EXTRA_QML"
+export EXTRA_QT_MODULES="svg"
+export EXTRA_PLATFORM_PLUGINS="libqwayland.so"
+export LDAI_OUTPUT="${LDAI_OUTPUT:-$BUILD_DIR/QtPotato-$(git -C "$SRC_DIR" describe --tags --always --dirty 2>/dev/null || echo dev)-x86_64.AppImage}"
+
+cd "$BUILD_DIR"
+linuxdeploy-x86_64.AppImage \
+    --appdir "$APPDIR" \
+    --executable "$APPDIR/usr/bin/qtpotato" \
+    --desktop-file "$APPDIR/usr/share/applications/dev.trivk.qtpotato.desktop" \
+    --icon-file "$APPDIR/usr/share/icons/hicolor/scalable/apps/dev.trivk.qtpotato.svg" \
+    --custom-apprun "$SRC_DIR/packaging/appimage/AppRun" \
+    "${deps_only[@]}" \
+    --plugin qt \
+    --output appimage
+
+echo "AppImage: $LDAI_OUTPUT"