Scattergl toself fill problem

[llnns]

Using scattergl with toself fill option results in a "ghost"/wrong/extra fill among the first points of each object. A minimal example creating three rectangles:

https://codepen.io/anon/pen/NBvbbv

Results in:

Also, using the same data with scatter type has the expected output:

[alexcjohnson]

Thanks for the nice simple reproduction @llnns - merging into #2291.