WebGPURenderer: add getInstanceIndex() for GLSLNodeBuilder (#26978)

* add getIinstanceIndex()

* adjust for type of builtin

---------

Co-authored-by: aardgoose <[email protected]>

examples/jsm/renderers/webgl/nodes/GLSLNodeBuilder.js

@@ -307,6 +307,12 @@ ${ flowData.code }
 
 	}
 
+	getInstanceIndex() {
+
+		return 'uint( gl_InstanceID )';
+
+	}
+
 	getFrontFacing() {
 
 		return 'gl_FrontFacing';