Fix #10242: Allow a space for text shadow when clipping WWT_EMPTY/WWT_TEXT.

[PeterN]

Motivation / Problem

See #10242.

Text drawn inside WWT_EMPTY or WWT_TEXT widgets may have cropped shadows if the widget height is not additionally padded by the widget.

Other widget types are already 'padded' by a bevel, so they are not affected.

Description

This is fixed by allowing extra vertical spacing when setting up the clipping area for a WWT_EMPTY or WWT_TEXT widget. There are still some conditions for this method to work, however it seems good enough in most cases.

Left is master, right is with this change applied.

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 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