Plot functions throw under GLMakie when is_save and is_show are both true #398

Under GLMakie, every plot function that saves and then shows fails. With is_save=true, is_show=true, save_plot attaches the figure's scene to a screen, and show_plot → display_named then opens a titled screen of its own, which throws:

GLMakie can not display a scene in multiple Screens.
  display_named(fig::Figure, name::String) @ ext/VortexStepMethodMakieExt.jl:29
  show_plot @ ext/VortexStepMethodMakieExt.jl:524
  plot_distribution(...) @ ext/VortexStepMethodMakieExt.jl:876

Reproduced on 675f0e7 (before #392) and on #392's branch, in a fresh session, with the same call: plot_distribution([y], [results], ["x"]; is_save=true, is_show=true, save_path=...) fails, and the same call with is_save=false works. examples/billowing.jl is the only example that sets SAVE_ALL = true, so it is the example menu.jl --run-all reports as failing.

The same save-then-show order is in plot_geometry, plot_polars, plot_combined_analysis and the polar-data plot. What I would do is show first and then save, or save from a figure that is not yet displayed, in one place (save_plot or show_plot) rather than in each plot function, plus a test that calls one plot function with both flags set under GLMakie. #391 edits the same functions, so this should land after it.