Function egui_fltk_frontend::begin_with

pub fn begin_with(
    window: &mut Window, 
    render_pass: RenderPass, 
    surface: Surface, 
    surface_config: SurfaceConfiguration, 
    scale: DpiScaling
) -> (Painter, EguiState)

Construct the frontend.

DpiScaling can be Default or Custom(f32)