* Added shaders on GLSL 1.2
- Used `glium::program` to create shaders
- Moved shaders code to its own sources and include it as str
- Added shaders implementation on GLSL which allows run egui on old hardware
(Raspberry Pi 1/zero in game again)
* Moved webgl shaders code to sources in `shader` subdir
* Added GLSL ES shaders to glium backend to support OpenGL ES
* Described changes related to GLSL versions support
