[all] Speed up spheres rendering + code cleaning (URGENT)

[fredroy]

Speed up by a (huge) factor the rendering of several spheres.
This is done in 2 ways:

• optimizing with VBO for "normal" spheres (like before, with rings and segments parameters)
• impostors (https://paroj.github.io/gltut/Illumination/Tutorial%2013.html)

On the same computer, for 150K spheres :

• 0.8fps with previous rendering
• 22fps with VBO
• 46 fps with impostors

This task is also the beginning of optimizing draw tools, by removing deprecated glu calls and non optimized OpenGL code ; and by using VBO/IBO and shaders. (class BasicShapesGL)

This PR should resolve more or less the issue #142 .
It depends if this issue talks about spheres only or more generally speeding up rendering itself (in that case, it will need much more commits ;) )

This PR:

• builds with SUCCESS for all platforms on the CI.
• does not generate new warnings nor unit test failures.
• does not break existing scenes.
• does not break API compatibility.
• has been reviewed and agreed to be transitional.
• is more than 1 week old.

Reviewers will merge only if all these checks are true.

[matthieu-nesme]

YEAH! Thanks Fred, do we have to wait for a week before merging?

[damienmarchal]

[ci-build]

[damienmarchal]

[ci-build]

[matthieu-nesme]

@fredroy It is working well, one minor problem is the spheres are darker, is it possible to tweak the shader to display them lighter by default? Thanks.

[fredroy]

You are right that the spheres are too dark ; I forgot to add the ambient component in the final color computation ; I will submit a patch right away.

[fredroy]

Pull-Request : #192