add support for preloading the icon_exists cache

[Absolucy]

About The Pull Request

uses https://github.com/absolucy/icon-presence-cache

Why It's Good For The Game

better performance

Changelog

no player-facing changes

[LikeLakers2]

As an aside - I might suggest putting this preloading functionality behind a compile-time flag... let's say PRELOAD_ICON_EXISTS_CACHE.

Then, if this flag is specified, we call into a new implementation of icon_exists() that uses the preloading - otherwise, we call into the old implementation:

#ifdef PRELOAD_ICON_EXISTS_CACHE

/proc/icon_exists(file, state)
	var/static/list/icon_states_cache = load_icon_exists_cache()
	var/file_name = "[file]"
	return !(isnull(icon_states_cache[file_name]?[state])

#else

/proc/icon_exists(file, state)
	// original implementation goes here

#endif

This avoids running code specific to the preloaded cache version, if we aren't using it (or otherwise don't want it) - and simultaneously, speeds up the preloaded cache version by not trying to populate the cache should the file not exist.

This, in turn, keeps icon_exists() as fast as possible in all cases - which is important, as icon_exists is a bit of a hot proc.

[LikeLakers2]

FWIW I wouldn't mind this being uncommented by default, if you think it'd be better.

[Absolucy]

currently there's not even a build step to do this, i'm doing this manually / via TGS build scripts