called by pack2x16unorm to convert to u32.
    var v: vec2<f32>;
    v[0] = textureLoadGeneral(src_tex, coord0, params.mipLevel).r;

    let coord1 = coord0 + vec3u(1, 0, 0);
    if (coord1.x < srcBoundary.x) {
        // Make sure coord1 is still within the copy boundary.
        v[1] = textureLoadGeneral(src_tex, coord1, params.mipLevel).r;
        result = pack2x16unorm(v);
    } else {
        // Otherwise, srcExtent.x is not a multiple of 2 and this thread is at right edge of the texture
        // To preserve the original buffer content, we need to read from the buffer and pack it together with other values.
        // TODO(dawn:1782): profiling against making a separate pass for this edge case
        // as it requires reading from dst_buf.
        let original: u32 = dst_buf[dstOffset];
        const mask = 0xffff0000u;
        result = (original & mask) | (pack2x16unorm(v) & ~mask);
    }