[FEATURE]: Add `fillpattern` support to `scattergl`

[Lexachoc]

Follow-up to #7923 and #8084.

scatter supports fillpattern, but scattergl does not.

It would be useful to add equivalent fillpattern support to scattergl.

This would also enable fillpattern support in scatterternarygl (#7058 ), which reuses scattergl.

[camdecoster]

Thanks for the idea! Any thoughts on how this could be implemented?

[Lexachoc]

I think it would be better to fix the existing scattergl fill bug (#7383) first, and then add this fillpattern feature on top of a working fill implementation. (otherwise, the tests would be difficult to validate)

fillpattern would still rely on the existing scattergl fill logic to construct the filled area, while the more complex part would be extending the WebGL fill shader to render patterns.