render selected plot for export offscreen

[kecnry]

This pull request addresses the issue where exporting figure widgets not rendered to screen fails by:

• rendering the selected plugin plot offscreen
• ensuring the plot's plugin is considered active when clicking the export button
• giving a small amount of time to ensure that the plot can update (but this is hardcoded and may not always be sufficient)

This "solution" still will not catch the case where the API is used to export a plot, but that same issue is known for the viewer plots as well and was present in the original export plot plugin.

[cshanahan1]

This solves the exporting plots when plugin is not active issue for me.

I don't know if this is a problem per se, but the highlight around an aperture will flash when exporting since the plugin is briefly active.

[kecnry]

I don't know if this is a problem per se, but the highlight around an aperture will flash when exporting since the plugin is briefly active.

Hmmm, good point. We could eventually make as_active more fine-grained to update plots but not show the indicators, or we could instead filter on active plugins instead of forcing it to be active when exporting. But for now, this might be the easiest path forward with the least downsides? I'll let you decide - and we can file follow-up tickets to refine anything remaining.

[cshanahan1]

I think filtering on active plugins would be very confusing. I personally think it fine to merge this as a fix, since the plugin is only very briefly active and it doesn't actually impact anything, and follow up with some fine tuning to make it more seamless.