Fix: JGRPP's NGRF chunk doesn't contain length indicators for fixed-size arrays

[TrueBrain]

Before TABLE chunks, SlArray (fixed-size arrays) weren't prefixed with a length indicator. But with the migration to TABLE chunks, they are. This is because otherwise there is no information in the savegame about the length of these fields, and would still require knowledge of the OpenTTD versions that created these chunks.

But, vanilla documented poorly that this was a change made, and with JGR's efforts in JGRPP to switch to TABLE chunks, this was overlooked. Now this doesn't show up when saving/loading games in-game, as everything is compatible. Although trying to load a JGRPP's savegame in Vanilla will also cause trouble.

Either way, BaNaNaS should just be able to load scenarios, no matter what OpenTTD produced them. As such, this proposed change.

It first detects a chunk that is JGRPP specific, which indicates what features are active. We use this for JGRPP to signal to us when the length indicator is there or not; for now the assumption is that table_newgrf_sl == 1 means it is not there, and that any newer versions do contain it. When version 1 is detected, ident.md5sum and param are hardcoded to their values as in JGRPP:

https://github.com/JGRennison/OpenTTD-patches/blob/9f7d25e02d5a4e86a88ebe9a6c83998fdcf7827e/src/sl/newgrf_sl.cpp#L83

PS: It has to be noted, although we detect this issue with the NGRF chunk, there might be other chunks effected. But this codebase only actually reads a very select few chunks, so not relevant for this repository.