Frontends, VideoCore: Move glad initialisation to the frontend

On SDL2 this allows it to use SDL_GL_GetProcAddress() instead of the
default function loader, and fixes a crash when using apitrace with an
EGL context.

On Qt we will need to migrate from QGLWidget to QOpenGLWidget and
QOpenGLContext before we can use gladLoadGLLoader() instead of
gladLoadGL(), since the former doesn’t expose a function loader.