Fix: Incorrect mask files for NE river slopes

[zephyris]

Motivation / Problem

A gray bar is visible when two or more NE facing river slopes are next to each other.

Description

The cause is an incorrect mask sprite for the NE river slopes. It is only visible when two or more tiles are next to each other as the river bank sprites normally cover the gray bar.

Limitations

None

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