fix getInstanceIndex()

it would be initialized after user material code was called.

shaders/src/main.vs

@@ -7,6 +7,12 @@
  */
 
 void main() {
+#if defined(TARGET_METAL_ENVIRONMENT) || defined(TARGET_VULKAN_ENVIRONMENT)
+    instance_index = gl_InstanceIndex;
+#else
+    instance_index = gl_InstanceID;
+#endif
+
     // Initialize the inputs to sensible default values, see material_inputs.vs
 #if defined(USE_OPTIMIZED_DEPTH_VERTEX_SHADER)
 
@@ -179,12 +185,6 @@ void main() {
     vertex_worldPosition.w = depth;
 #endif
 
-#if defined(TARGET_METAL_ENVIRONMENT) || defined(TARGET_VULKAN_ENVIRONMENT)
-    instance_index = gl_InstanceIndex;
-#else
-    instance_index = gl_InstanceID;
-#endif
-
     // this must happen before we compensate for vulkan below
     vertex_position = gl_Position;