Fix: Add shading to river slopes

[zephyris]

Adds a 32bpp shading to the river rapids/slopes to make them more visible. Fixes #9031

Motivation / Problem

Water slopes on rivers are not readily visible as sloped, making it confusing why ships cannot sail through them.

Description

This fix adds 32bpp sloped river water sprites to the orig_extra.grf. By using the 8bpp sprite as a mask, this adds shading to the slope while retaining the animation and 8bit style. Requires a bump of original graphics grfs to use container version 2.

Limitations

Does not work with 8bpp blitters.

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, game_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

[TrueBrain]

This is very cool, and will really help me when playing with Rivers. Thank you for this :D

[PeterN]

Can't read sprites/rapids_shading.png

Existing bug I guess, the project needed to be reconfigured, it would not copy this file otherwise.

[PeterN]

I think it's acceptable that it does not work on 8bpp -- that would require some changing the palette animation definitions to use otherwise unused palette entries, which could cause compatibility issues. Nothing is broken with 8bpp, you just need to use 32bpp to see the difference.

Whereas this solution already works (even if I'd forgotten I'd made it possible...!) and just needs the graphics.