Codechange: Make GetCurrentRect() conform to usual Rect bounds, and reuse it.

[PeterN]

Motivation / Problem

Similar code to GetCurrentRect() is already repeated in other locations, but Rect's right and bottom should refer to the edge of the widget, not the outside.

Description

GetCurrentRect() is tweaked and used in place of some of the more obvious uses.

Limitations

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