Returns true if this thread is either running or set to be running the simpledisplay.d gui core event loop because it owns windows.

It is important to keep gui-related functionality in the right thread, so you will want to runInGuiThread when you call them (with some specific exceptions called out in those specific functions' documentation). Notably, all windows must be created and managed only from the gui thread.

Will return false if simpledisplay's other functions haven't been called yet; check guiThreadExists in addition to this.