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/webgl1.rs

@@ -538,6 +538,10 @@ impl crate::Painter for WebGlPainter {
 
         Ok(())
     }
+
+    fn name(&self) -> &'static str {
+        "egui_web(webgl1)"
+    }
 }
 
 struct PostProcess {