Merge: lib/glesv2: intro framebuffer related services and try a new style

The main point of this PR is to try a new naming convention for the glesv2 module.

Ideas behind this convention:
1. Be as close as possible to the C API: use the same function and macro names.
2. Modify those names only for compatibility with the Nit language: the macros cannot begin with `GL_` so use `gl_` instead.
3. Apply types and other Nity features when they can help and don't interfere with the original API.

It differs from the previous convention of renaming all entities to the Nit style. It is not heavier in the module definition but should be easier for the users.

In practice, to convert OpenGL ES C code to Nit you would need to change the prefix `GL_` to `gl_`, and remove `;`.

In C:
~~~
glBindFramebuffer(GL_FRAMEBUFFER, my_fbo);
glBindRenderbuffer(GL_RENDERBUFFER, my_renderbuffer);
return glCheckFramebufferStatus(GL_FRAMEBUFFER) == GL_FRAMEBUFFER_COMPLETE;
~~~

In Nit:
~~~
glBindFramebuffer(gl_FRAMEBUFFER, my_fbo)
glBindRenderbuffer(gl_RENDERBUFFER, my_renderbuffer)
return glCheckFramebufferStatus(gl_FRAMEBUFFER) == gl_FRAMEBUFFER_COMPLETE
~~~

What do you think of the new naming convention? I would convert the whole module to it.

Pull-Request: #1157
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>