Use GetProcAddress to fetch GL core symbols instead of using symbols from the
libGL linked with the vlc gl plugin (but not for GLES2/Windows/macOS).

This fixes a crash with nvidia drivers when using EGL but can also fix some
others issues if a vendor/distrib ship more than one openGL libs.

cf. https://github.com/NVIDIA/libglvnd/issues/63

Symbols from the old libGL.so.1 were used instead of symbols from the new
libOpenGL.so (that are fetched via eglGetProcAddress()).

I guess that libGL.so.1 can't work with EGL with recent drivers.

Fixes #19093