Fix some missing files and inability to find others

[lucyllewy]

• Add some extra layout definitions for gimp to find some platform executables and shared files
• Update the environment: block for runtime search paths that are missing in the platform snap's desktop-launch script
• Add missing libva-drm and libva-x11 libraries
• Ensure gimp is built as a relocatable executable so that it can find some of its assets easier in the snap
• Move the debug symbols compiler flags to use the configure script's support
• Add missing iso-codes because gimp refuses to look for them anywhere but $SNAP/usr/share/xml/iso-codes
• Add missing lua-lgi for gimp's lua scripting capability
• Fix the cleanup part to not delete the iso-codes we're including in the snap directly

Signed-off-by: Dani Llewellyn [email protected]

[merlijn-sebrechts]

(FYI closed and re-opened pull request because I don't seem to have permissions to rerun tests otherwise)

[kenvandine]

Here's the relevant error from the build failure to avoid others needing to try to view the massive build log.

2023-10-18T20:34:39.5587919Z :: + autoreconf --install
2023-10-18T20:34:39.6364284Z :: autoreconf: error: 'configure.ac' is required
2023-10-18T20:34:39.6376076Z :: error: Failed to run the build script for part 'gimp'.
2023-10-18T20:34:39.8404658Z 'override-build' in part 'gimp' failed with code 1.

[jnsgruk]

I've taken the changes from here and added them in #255 - closing this for now to avoid a messy merge. Thanks @lucyllewy!