Added unit tests for Java, JavaScript and JSON mods

core/src/mindustry/graphics/CacheLayer.java

@@ -3,6 +3,7 @@ package mindustry.graphics;
 import arc.*;
 import arc.graphics.*;
 import arc.graphics.gl.*;
+import arc.util.*;
 
 import static mindustry.Vars.*;
 
@@ -53,7 +54,7 @@ public class CacheLayer{
     }
 
     public static class ShaderLayer extends CacheLayer{
-        public Shader shader;
+        public @Nullable Shader shader;
 
         public ShaderLayer(Shader shader){
             //shader will be null on headless backend, but that's ok