Fix gizmos (#15836)

# Objective

- Immediate mode gizmos don't have a main world entity but the phase
items require `MainEntity` since #15756

## Solution

- Add a dummy `MainEntity` component.

## Testing

Both the `3d_gizmos` and `2d_gizmos` examples show gizmos again

crates/bevy_gizmos/src/lib.rs

@@ -93,6 +93,7 @@ use crate::config::GizmoMeshConfig;
 use {
  bevy_ecs::{
  component::Component,
+ entity::Entity,
  query::ROQueryItem,
  system::{
  lifetimeless::{Read, SRes},
@@ -109,7 +110,7 @@ use {
  ShaderStages, ShaderType, VertexFormat,
  },
  renderer::RenderDevice,
- sync_world::TemporaryRenderEntity,
+ sync_world::{MainEntity, TemporaryRenderEntity},
  Extract, ExtractSchedule, Render, RenderApp, RenderSet,
  },
  bytemuck::cast_slice,
@@ -467,6 +468,9 @@ fn extract_gizmo_data(
  render_layers: config.render_layers.clone(),
  handle: handle.clone(),
  },
+ // The immediate mode API does not have a main world entity to refer to,
+ // but we do need MainEntity on this render entity for the systems to find it.
+ MainEntity::from(Entity::PLACEHOLDER),
  TemporaryRenderEntity,
  ));
  }