feat: draw custom background behind fresk

Description

It's currently impossible to draw background behind fresks. It is required for the "fresque des frontières planétaires".

Issues : Closes #141 (closed). Closes fresque-des-frontieres-planetaires#2 (closed)

Documentation

• Upgrade the version
• Add entry to the changelog
• Update documentation if needed

Generator

• New or updated files?

Checklist before merge :

• No remaining console.log
• Version changed if new feature
• Lint ok
• all eslintConfig rules are on ("2") (except no-useless-escape / vue/no-mutating-props)
• Test aktivisda and backtivisda

Tests

Not relevant yet