Added shaders on GLSL 1.2

[katyo]

• 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)

It should solve #79 #80.
Another shader versions (1.1, 1.3, 1.0 ES and etc.) still missing and can be added later.

[katyo]

I'm not an expert in GLSL dialects but seems the shader's code from webgl1 and webgl2 can be reused for GLSLES 1.0 and 3.0 respectively.

[emilk]

Awesome, thank you! Please also add a line to egui_glium/CHANGELOG.md

[emilk]

@katyo did you test both fragment_300es.glsl and fragment_100es.glsl ?

I ask because they seem to be carbon-copied of the WebGL shaders, including statements like "WebGL doesn't come with sRGBA textures" (but does GLES?) and "WebGL doesn't support linear blending in the framebuffer" (but does GLES?).

This is particularly relevant since I recently improved the WebGL shaders: effd3c7

[katyo]

@emilk Seems the version string is wrong for GLSL ES 1.00 shaders. According to specs it should be #version 100 without es at end. I forget to fix this.
I cannot test GLSL ES 3.0 version just now, but 1.00 works fine (with version fix).
This wiki page has tables with GLSL versions and corresponding version strings.