Fix mesh tag feature for 2d. (#18636)

# Objective

Fixes #18564

Author: tychedelia

crates/bevy_sprite/src/mesh2d/mesh2d_functions.wgsl

@@ -43,3 +43,7 @@ fn mesh2d_tangent_local_to_world(world_from_local: mat4x4<f32>, vertex_tangent:
         vertex_tangent.w
     );
 }
+
+fn get_tag(instance_index: u32) -> u32 {
+    return mesh[instance_index].tag;
+}

crates/bevy_sprite/src/mesh2d/mesh2d_types.wgsl

@@ -13,4 +13,5 @@ struct Mesh2d {
     local_from_world_transpose_b: f32,
     // 'flags' is a bit field indicating various options. u32 is 32 bits so we have up to 32 options.
     flags: u32,
+    tag: u32,
 };