[EGL] Fix initialization and extension loading

This PR fixes two issues affecting the EGL backend:
- eglGetProcAddress can only be used with extension functions, not core functions. OpenTK will now use the native symbol loader for core functions (GetProcAddress on windows, dlsym on Unix). This fixes crashes on various EGL implementations.
- OpenTK will now use the display connection specified in EglWindowInfo throughout the EGL backend. This fixes a failure in eglCreateWindowSurface on Linux/Mesa.