Workaround for scattergl now always putting selected points on top layer

See https://github.com/plotly/plotly.js/issues/2298

```r
library(crosstalk)
d <- SharedData$new(mtcars)
p <- plot_ly(d, x = ~mpg, y = ~wt)
subplot(p, p) %>% toWebGL() %>% highlight("plotly_selected")
```

