egl: adjust how we load symbols in SDL_EGL_GetProcAddress. Use eglGetProcAddress for everything on EGL >= 1.5. Try SDL_LoadFunction first for EGL <= 1.4 in case it's a core symbol, and as a fallback if eglGetProcAddress fails. Finally, for EGL <= 1.4, fallback to eglGetProcAddress to catch extensions not exported from the shared library. (Maybe) Fixes Bugzilla #4794.