Add: show fallback sprites with debuglevel sprite=4

[Eddi-z]

Motivation / Problem

When trying to update a baseset, only the number of missing sprites is shown, not which sprites are missing, which may make the process unnecessarily complicated.

Description

This adds debuglevel sprite=4 which lists all sprites taken from the fallback grf. while here, bump lru debug messages to level 5, because they seem to be rather spammy and unhelpful for this usecase.

Limitations

The original baseset makes intentional use of the fallback, which is a lot spammier than it needs to be.

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 touches english.txt or translations? Check the guidelines
• 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