Motivation / Problem

Whilst debugging window layouts I noticed that there some places where whole windows are marked dirty when in fact only one widget needs to be drawn. This is not really an issue but I had an itch.

Description

Some actions that previously marked a window dirty now only mark the relevant widget dirty.
Scrollbar behaviour has been tweaked so that if the scrollbar position is not changed, no update is performed.

Limitations

Fairly sure this is no where near all cases...

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