Incorporate opengles support

[OptimisticPeach]

Hi there, I have been using opengles_graphics as of late, and have noticed that it is rather old (2 years ago was the last commit), and so I was wondering if opengles support could be added to opengl_graphics. Currently I use a fork of opengles_graphics (So a fork of a fork of this repo) which allows me to use newer libraries in my android/opengles projects, but it's unwieldy to work with, not to mention that I'm not going to maintain my fork of opengles_graphics because of my inexperience in opengl (much less opengles).
Thanks!

[agausmann]

In my experience, opengl_graphics works fine with OpenGL ES 2.1/3.0. Have you tried using this crate instead?

---

E: My use case wasn't GL ES per se, but it was the emulated layer in Emscripten. There is technically a difference in that it uses different shaders (Emscripten uses WebGL in the backend and hence requires WebGL shaders), and it appears that opengles_graphics exists for a similar reason.

I'd still be interested in seeing what specific errors are raised by running this crate on OpenGL ES. My guess is that they'll be related to shader compilation.

[OptimisticPeach]

A few things actually, since we're targeting OpenGLES 3.1, the bindings change and break some things, and the shaders must be rewritten using a different kind of glsl:

// Vertex Shader
#version 300 es
in vec4 color;
in vec2 pos; 

out vec4 v_Color;

void main() {
    v_Color = color;
    gl_Position = vec4(pos, 0.0, 1.0);
}

// Fragment Shader
#version 300 es
precision mediump float;

in vec4 v_Color;
out vec4 outColor;

void main() {
    outColor = v_Color;
}

Which happens to not be what opengles_graphics does.

By the way, opengles_graphics is ancient, and it seems the author agrees and has archived the repo.

I don't have a reproducible build of using opengl_graphics when OpenGLES is requested currently but I'll make one in the near future.

[agausmann]

Which happens to not be what opengles_graphics does.

As far as I could tell, the majority of changes involve replacing shader-related code.