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/webgl2.rs

@@ -516,6 +516,10 @@ impl crate::Painter for WebGl2Painter {
 
         Ok(())
     }
+
+    fn name(&self) -> &'static str {
+        "egui_web(webgl2)"
+    }
 }
 
 /// Uses a framebuffer to render everything in linear color space and convert it back to sRGB