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/lib.rs

@@ -15,6 +15,8 @@
 #![warn(clippy::all, missing_crate_level_docs, rust_2018_idioms)]
 
 pub mod backend;
+#[cfg(feature = "glow")]
+mod glow_wrapping;
 mod painter;
 pub mod screen_reader;
 pub mod webgl1;