Change: Shortcut varaction chains for callbacks.

[PeterN]

Motivation / Problem

By design, an unhandled callback often ends up following the default graphics chain. Depending on the variables accessed this could potentially be expensive.

Description

If a varaction chain does contain any callback results, do not continue evaluating the chain. This possibly avoids processing complex graphical chains which can never end in a callback result.

Limitations

This is not really benchmarked and is hypothetical at this point. It's not impossible that the additional test causes more work over all.

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, gs_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker

[frosch123]

This looks like it need simliar checks as NML's switch optimisation.

• Rerandomisation is already part of the diff.
• Writing persistent storage is missing.

[PeterN]

Added what I think is the case for writing to persistent storage. Not really familiar with that system tbh.