Feature: Press ctrl to build diagonal rivers in Scenario Editor

[2TallTyler]

Motivation / Problem

This is a JGRPP patch, created by @JGRennison. I am simply upstreaming it.

Description

In Scenario Editor, pressing ctrl makes a diagonal selector for the river tool — just like the Bomb tool.

The canal tool is unchanged, both in SE and in-game.

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

[LordAro]

I like it

[Milek7]

Tile iterator is leaked.