Take Glow context using Arc. (#1640)

This allows usage with a Glow context that is passed between threads.

eframe/src/native/run.rs

@@ -51,7 +51,7 @@ pub fn run_glow(
     let window_builder =
         epi_integration::window_builder(native_options, &window_settings).with_title(app_name);
     let (gl_window, gl) = create_display(native_options, window_builder, &event_loop);
-    let gl = std::rc::Rc::new(gl);
+    let gl = std::sync::Arc::new(gl);
 
     let mut painter = egui_glow::Painter::new(gl.clone(), None, "")
         .unwrap_or_else(|error| panic!("some OpenGL error occurred {}\n", error));