Shader getShaderFi check for mod files (#9599)

* Shader getShaderFi check for mod files

* Just use tree.get()

It defaults to files.internal() if nothing is found. Actually, I think this also has the added benefit of allowing mods to override vanilla shaders by having a frag with an identical name.

core/src/mindustry/graphics/Shaders.java

@@ -474,6 +474,6 @@ public class Shaders{
     }
 
     public static Fi getShaderFi(String file){
-        return Core.files.internal("shaders/" + file);
+        return tree.get("shaders/" + file);
     }
 }