[MIRROR] Adds lazyloading to the asset subsystems [MDB IGNORE] (#15960)

* Adds lazyloading to the asset subsystems (#69454)

* Adds lazyloading to the asset subsystems

This currently applies only to spritesheets because of how monumentally
expensive they are.
If an asset is requested it will immediately be fully loaded, but
otherwise we slowly load them in with a separate subsystem.

This allows us to not hold up initialize with hair stuff. Saves roughly
33% (16 seconds with LOW_MEMORY_MODE) of initialize on my machine

My target is something closer to the 9 second init that had back in
2019, this is a good first step. Lets see how much more we can do yeah
lads?

Co-authored-by: san7890 <the@ san7890.com>

* Adds lazyloading to the asset subsystems

Co-authored-by: LemonInTheDark <58055496+LemonInTheDark@users.noreply.github.com>
Co-authored-by: san7890 <the@ san7890.com>

code/controllers/subsystem/early_assets.dm

@@ -16,7 +16,7 @@ SUBSYSTEM_DEF(early_assets)
 		if (!initial(asset_type.early))
 			continue
 
-		if (!get_asset_datum(asset_type))
+		if (!load_asset_datum(asset_type))
 			stack_trace("Could not initialize early asset [asset_type]!")
 
 		CHECK_TICK