Make egui_glow painter to work on web (#868)

Add WebGL1 and WebGL2 support to glow painter.
Add "glow" feature to egui_web to use the glow painter there.
Make winit an optional part of egui_glow

egui_web/src/painter.rs

@@ -17,4 +17,6 @@ pub trait Painter {
         clipped_meshes: Vec<egui::ClippedMesh>,
         pixels_per_point: f32,
     ) -> Result<(), JsValue>;
+
+    fn name(&self) -> &'static str;
 }