Update gimp, migrate to core24

[jnsgruk]

This is a bit of maintenance to ensure we're ready when GIMP 3.0 releases.

• Bumps gmic, which enables us to drop a patch I wrote last time
• Bumps rawtherapee, enabling us to drop another patch!
• Grab gimp from master for now, but moving to 2.99.19-rc soon

Edit: subsequent patches...

• Drop rawtherapee
• Use libwmf & ghostscript from the archive
• Cleanup some explicit things taken care of by the gnome extension
• Remove evince (I can't see why we're shipping this in the snap?)

Edit (again!):

• Migrate to core24 and use more libraries from the archive

[lucyllewy]

LGTM

[jnsgruk]

Some updates (I've edited the PR description):

• Drop rawtherapee
• Use libwmf & ghostscript from the archive
• Cleanup some explicit things taken care of by the gnome extension
• Remove evince (I can't see why we're shipping this in the snap?)

[lucyllewy]

LGTM

[brunvonlope]

Remove evince (I can't see why we're shipping this in the snap?)

@jnsgruk This dep is there probably because of the custom print preview

[jnsgruk]

Remove evince (I can't see why we're shipping this in the snap?)

@jnsgruk This dep is there probably because of the custom print preview

Ah interesting. I'll update to just stage it from the archive then.

[jnsgruk]

This built, but didn't actually run due to a mismatch in libharfbuzz versions. I'm in the process of migrating the snap to 24.04, which should fix that.

[jnsgruk]

Updated to core24, but broke gmic in the process, will attempt to fix that next. So far these changes have made quite a nice simplification to the codebase, and also reduced the size of the snap by over 300MB!

[jnsgruk]

Last little hurdle now before this is ready! I get this error when I try to start Gmic...

If anyone knows how to solve this, I'm all ears... perhaps @kenvandine or @sergiusens?

[jnsgruk]

G'mic working! Will merge this so that a candidate build gets pushed that can be tested more easily by folks!