Motivation / Problem

See #8874. Dumping many rescannewgrf commands onto the console will, in the old situation, drop many rescans without notifying the user.

Description

Check whether a NewGRF scan is running before actually requesting a new one. The function to request the scan will return true when a scan was triggered and false when one was already running. In case one was already running, show a warning.

Limitations

Really want to know? ;)

• the UI is blocked while rescanning, so the suggestion from Eddi-z cannot be done as either the scanning has not started and then there is no need to stop, or the scanning was already done and it will trigger a new scan.
• now the callback of the first rescan is passed on, instead of the callback of the last rescan request in that tick. This could be solved by making a list of callback, but comes with its own can of worms, and even then someone clicking in the UI and rescanning from the console in the same tick seems unlikely.
• there is a new message on the console that might mess with bots processing it.

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