glow painter every where

[KentaTheBugMaker]

egui_web webgl1 and webgl2 use almost same code .
egui_glow painter use very similar code to web painters.
so i port PostProcess to egui_glow.
and create new crate that only contains painter and context create utility.
Current status

• OpenGL 2.1 or above(same as glow and glium)
• WebGL2(same as egui_web)
• WebGL1(same as egui_web)
• WebGL1 + sRGB(same as egui_web)

[KentaTheBugMaker]

eliminated VAO by emulating VAO in painter.

[emilk]

What is the motivation for adding egui_glow_painter as a separate crate? If you don't want the winit-stuff of egui_glow, perhaps we can hide that behind a feature-flag instead so we don't get too many crates?

[KentaTheBugMaker]

OK I will make more acceptable new PR.