separate shader defines for each pipeline

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https://github.com/mosure/bevy_gaussian_splatting/blob/4356f87a7a5353e33e997297ed96714d11cdc4be/src/render/mod.rs#L360

            ],
        });

        GaussianCloudPipeline {
            gaussian_cloud_layout,
            gaussian_uniform_layout,
            view_layout,
            shader: GAUSSIAN_SHADER_HANDLE,
            sorted_layout,
        }
    }
}

// TODO: allow setting shader defines via API
// TODO: separate shader defines for each pipeline
struct ShaderDefines {
    radix_bits_per_digit: u32,
    radix_digit_places: u32,