scattergl image background feature request

[dirtyredz]

I was using scatter until my graph's data grew to over 25k points. When I transfered to scattergl for performance. The background image I was using did not appear. The image for the graph and the zooming ability is integral to the experience I'm providing.

Ive been unable to find any info available to whether or not images are supported for scattergl. Any information or support for this would be appreciated.

[etpinard]

cc @dfcreative would be nice in scattergl v2

[etpinard]

Will be completed in @dfcreative 's regl-based scattergl:

we should add a test mock before merging.

[dy]

Added test mock in 20491b2

[etpinard]

Done in #2258