make bevy_pbr optional in bevy_gizmos_render again

[ChristopherBiscardi]

Objective

https://github.com/bevyengine/bevy/pull/22443/changes#diff-0425515a2f1ec5b53401db2850b549c2d6634bc1c3e2ae9698d0ccfb172f2387R36 introduced a hard dependency on bevy_pbr to bevy_render_gizmos, which is marked as optional and will not compile if the optional dep is not enabled:

error[E0432]: unresolved import `bevy_pbr`
  --> /Users/chris/github/bevyengine/bevy-clean/crates/bevy_gizmos_render/src/lib.rs:36:5
   |
36 | use bevy_pbr::MeshPipelineSet;
   |     ^^^^^^^^ use of unresolved module or unlinked crate `bevy_pbr`
   |

Solution

hide it behind the same bevy_pbr flags as the other bevy_pbr code

Testing

Use bevy_gizmos_render as a path dependency in a freshly cargo new'd project.

[atlv24]

good catch but also yuck, this shouldnt hard or soft dep on pbr at all. not sure it can be untangled until Charlotte's work lands though

[alice-i-cecile]

We should only be cfg gating the ordering, not the addition of the system.

You can use configure_sets to do this nicely.

[ChristopherBiscardi]

@alice-i-cecile Upon further inspection of the functionality added in the original PR, the only reason I can see that this would need to be ordered is if it accessed MeshPipelineViewLayouts, which it does not do. So I have removed the cfg gating and the bevy_pbr usage that was added in the PR.

@Zeophlite Was there another reason this system was ordered against MeshPipelineSet that I'm missing?

fn init_line_gizmo_uniform_bind_group_layout(mut commands: Commands) {
    let line_layout = BindGroupLayoutDescriptor::new(
        "LineGizmoUniform layout",
        &BindGroupLayoutEntries::single(
            ShaderStages::VERTEX,
            uniform_buffer::<LineGizmoUniform>(true),
        ),
    );

    commands.insert_resource(LineGizmoUniformBindgroupLayout {
        layout: line_layout,
    });
}

[ChristopherBiscardi]

The system that needed ordering was init_line_gizmo_pipelines (a different system from the original mentioned in the PR), which accesses Res<MeshPipeline>, which is set up as part of MeshPipelineSet. That ordering can happen in the pipeline_3d where the entire module is already gated by bevy_pbr.