Set this to true (e.g. commands.insert_resource(AppExit(true))) to
stop the default headless polling loop after the current tick. Has no
effect on a windowing plugin’s own runner — closing the window is what
stops that one.
Whether the GPU backend has finished initializing. false until a
plugin (e.g. GraphicsPlugin) acquires one and flips it — until then,
App::update only runs gpu_schedules, not the regular stages.