Stops gas overlays from inheriting turf color (#2473)

## About The Pull Request
Adds RESET_COLOR appearance_flag to stop gas overlays from inheriting
turf color
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## Changelog

:cl:
add: Gas overlays should now look normal on turfs with a color var set.
/:cl:

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code/modules/atmospherics/gasmixtures/auxgm.dm

@@ -138,7 +138,7 @@ GLOBAL_DATUM_INIT(gas_data, /datum/auxgm, new)
 	mouse_opacity = MOUSE_OPACITY_TRANSPARENT
 	anchored = TRUE  // should only appear in vis_contents, but to be safe
 	layer = FLY_LAYER
-	appearance_flags = TILE_BOUND
+	appearance_flags = TILE_BOUND | RESET_COLOR
 	vis_flags = NONE
 
 /obj/effect/overlay/gas/New(state)