IconForge: rust-g GAGS (250x faster edition) (#89590)

## About The Pull Request

Offloads GAGS generation to rust-g IconForge.

**Key Notes**

- The builtin GAGS editor still uses the 'legacy' generation to allow
for debugging.
- Does not support `color_matrix` layer type, which is currently not
used by any GAGS configs. Will do nothing if used.
- Does not support `or` blending mode, which is currently not used by
any GAGS configs. Will error if used.
- Has some 'quirks' compared to BYOND when it comes to mixing icon
states with different dir/frame amounts. BYOND will just silently handle
these and it's basically undefined behavior because what should you
expect BYOND to do? IconForge will spit errors out instead. So this PR
also fixes a few of those cases.

Functions by writing output to `tmp/gags/gags-[...].dmi`, copying that
output into the RSC and assigning the file object to `icon`.

Saves ~1.7s init by reducing worst-case GAGS icon generation from 250ms
to 1ms.

Also optimizes `icon_exists` by using `rustg_dmi_icon_states` for file
icons, saving ~60ms. Would have more savings if not for json_decode as
well as DMI parsing in rust being somewhat slow. Perhaps having
`rustg_dmi_icon_states` share a cache with IconForge could reduce this
cost, however I'd still recommend limiting these tests to unit tests
(https://github.com/tgstation/dev-cycles-initiative/issues/34),
especially for GAGS configs. I'm not sure they're worth 700ms.

Saves another ~400ms by replacing `md5asfile` with `rustg_hash_file` in
`/datum/greyscale_config/proc/Refresh`

Savings are likely even higher when combined with #89478, due to
spritesheets sharing a parsed DMI cache with GAGS. This means GAGS will
spend less time parsing icons synchronously and can generate output
faster. Tracy tests with this combo seem to yield ~2sec savings instead
of ~1.7sec

Total savings: ~2.16sec to ~2.46sec

- Ports https://github.com/BeeStation/BeeStation-Hornet/pull/10455
- Resolves https://github.com/tgstation/dev-cycles-initiative/issues/9



## Why It's Good For The Game

GAGS go zoooom

<details>
<summary>GAGS Working Ingame</summary>


![image](https://github.com/user-attachments/assets/28df25a5-bdf0-4a63-a6cf-b15f85467b23)


![image](https://github.com/user-attachments/assets/6a9dab46-5814-47ea-ad9b-f5ec84a6333d)

</details>

<details>
<summary>GetColoredIconByType</summary>


![image](https://github.com/user-attachments/assets/1698729f-1101-4413-bfb3-0922b389c347)

</details>

<details>
<summary>icon_exists</summary>


![image](https://github.com/user-attachments/assets/9e72c6aa-287f-4ce3-8dbe-9d3bebf3a762)

</details>


<details>
<summary>Refresh</summary>


![image](https://github.com/user-attachments/assets/18c15073-a294-4db6-bdd0-cdc7d8682221)

</details>


## Changelog
🆑
tweak: Optimized GAGS using rust-g IconForge, reducing worst-case
generation time to 1ms
/🆑

code/modules/unit_tests/unit_test.dm

@@ -136,15 +136,18 @@ GLOBAL_VAR_INIT(focused_tests, focused_tests())
 		var/data_filename = "data/screenshots/[path_prefix]_[name].png"
 		fcopy(icon, data_filename)
 		log_test("\t[path_prefix]_[name] was found, putting in data/screenshots")
-	else if (fexists("code"))
-		// We are probably running in a local build
-		fcopy(icon, filename)
-		TEST_FAIL("Screenshot for [name] did not exist. One has been created.")
 	else
-		// We are probably running in real CI, so just pretend it worked and move on
+#ifdef CIBUILDING
+		// We are runing in real CI, so just pretend it worked and move on
 		fcopy(icon, "data/screenshots_new/[path_prefix]_[name].png")
 
 		log_test("\t[path_prefix]_[name] was put in data/screenshots_new")
+#else
+		// We are probably running in a local build
+		fcopy(icon, filename)
+		TEST_FAIL("Screenshot for [name] did not exist. One has been created.")
+#endif
+
 
 /// Helper for screenshot tests to take an image of an atom from all directions and insert it into one icon
 /datum/unit_test/proc/get_flat_icon_for_all_directions(atom/thing, no_anim = TRUE)