opengl: use GL core symbols from *GetProcAddress
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