benefit from 32-bit opacity

[github-actions[bot]]

https://github.com/mosure/bevy_gaussian_splatting/blob/eb0fcc33900f7ad99357d7251f9b5da4c1cab89b/src/gaussian/f16.rs#L165

}

#[derive(
    Clone,
    Debug,
    Default,
    Copy,
    PartialEq,
    Reflect,
    ShaderType,
    Pod,
    Zeroable,
    Serialize,
    Deserialize,
)]
#[repr(C)]
pub struct Covariance3dOpacityPacked128 {
    #[reflect(ignore)]
    pub cov3d: [u32; 3],
    pub opacity: u32,
}

impl Covariance3dOpacityPacked128 {
    pub fn from_gaussian(gaussian: &Gaussian) -> Self {
        let cov3d: Covariance3dOpacity = gaussian.into();
        let cov3d = cov3d.cov3d;

        let opacity = gaussian.scale_opacity.opacity;

        Self {
            cov3d: [
                pack_f32s_to_u32(cov3d[0], cov3d[1]),
                pack_f32s_to_u32(cov3d[2], cov3d[3]),
                pack_f32s_to_u32(cov3d[4], cov3d[5]),
            ],
            opacity: pack_f32s_to_u32(opacity, opacity),  // TODO: benefit from 32-bit opacity
        }
    }

    pub fn covariance_3d_opacity(&self) -> Covariance3dOpacity {
        let (c0, c1) = unpack_u32_to_f32s(self.cov3d[0]);
        let (c2, c3) = unpack_u32_to_f32s(self.cov3d[1]);
        let (c4, c5) = unpack_u32_to_f32s(self.cov3d[2]);

        let (opacity, _) = unpack_u32_to_f32s(self.opacity);

        let cov3d: [f32; 6] = [c0, c1, c2, c3, c4, c5];

        Covariance3dOpacity {
            cov3d,
            opacity,
            pad: 0.0,
        }
    }
}

impl From<[u32; 4]> for Covariance3dOpacityPacked128 {
    fn from(cov3d_opacity: [u32; 4]) -> Self {
        Self {
            cov3d: [
                cov3d_opacity[0],
                cov3d_opacity[1],
                cov3d_opacity[2],
            ],
            opacity: cov3d_opacity[3],
        }
    }
}

pub fn pack_f32s_to_u32(upper: f32, lower: f32) -> u32 {
    pack_f16s_to_u32(
        f16::from_f32(upper),