Fix #7847: Use ViewportSign coordinates for sign Kdtree coordinates (#7849)

Ensure the same coordinates are used for station/town/player signs regardless of how the landscape changes below it after the coordinates were first determined.

By keeping track of whether each ViewportSign is valid for Kdtree use (and only ever registering the viewport sign when the object is valid) a lot of code can be simplified and become more robust at the same time.

Revert #7837, 7e22f243e: OpenTTD tries to replicate the original game mechanics.
Making numbers look nice and correlate is no goal, and in this case they actually did not correlate.

Add #6887: Option to show zone inside local authority boundary of towns

Can be found at town information > local authority window
Layout for button is same as Graph Keys
Turn on/off for every town individually

Add: Option for population-linear town cargo generation

Introduce a new default algorithm for town cargo generation (passengers and mail), and a game setting to choose between the new and original algorithm.

The original town cargo generation algorithm has the property of the generated amount relating to the square of each building's population, meaning large towns easily produce more cargo than can realistically be transported. The problem is excessive cargo is amplified if playing with cargodist.

The new algorithm introduced instead has a linear relation to the population. The result is that smaller towns will produce slightly more cargo, while the largest towns will produce about a fourth of what they would with the original algorithm.

Existing savegames will use the original algorithm, while new games will default to the new algorithm.

Fix #7043, Fix #7274: Delete town owned bridge based on adjacent tile ownership, not nearest town. (#7284)

This only affects failed town generation, as towns do not delete bridges under any other circumstances.

The existing test performed badly with a large number of towns due to having to calculate the
nearest town, and also by its nature assumed a bridge was built by the nearest town, leading
to bridges of nearby large towns be removed incorrectly.

If we gain the ability to quickly retrieve the correct town (which is _not_ the nearest town) from the bridge, this change should be reviewed.

Add: Houses can accept up to 16 different cargo types via NewGRF.

New Action0 property 23 for feature 07, variable length, format B n*(B B). Initial byte is number of structures following. First byte in structure is cargo id, second is acceptance level in 1/8 units.

Fix: Some code and comment typos

Found with codespell

Feature #6397: Keep town growth rate in sync with house count

Takes some code and ideas from #6378 patch, but doesn't change anything GS-related.