Migrate from GLEW to libepoxy or GLAD

Hi,

we are currently trying to get rid of GLEW in Fedora as it isn't [maintained](https://github.com/nigels-com/glew/issues/416#issuecomment-2646570907) anymore. Also GLEW with the last release only supports GLX and not EGL.

There are two ways moving forward:
* Use [libepoxy](https://github.com/anholt/libepoxy)
* Use [GLAD](https://github.com/Dav1dde/glad/), or check out the [webservice for glad2](https://glad.sh/) to generate the files you need!

With both of them migration should be straight forward.

Thanks for considering!

L’issue mentionne GLEW, libepoxy et GLAD, mais ne fournit aucun fichier, test ou point d’entrée. Commencez par localiser l’utilisation de GLEW et la configuration du build, puis déterminez quelle option de migration convient à OpenColorIO ; le travail est considéré comme terminé lorsque GLEW n’est plus requis et que le comportement pertinent de GLX/EGL est couvert.

Rédigé par le modèle d'indexation à partir du texte de l'issue.