wayland: check for egl extensions explicitly

wayland: check for egl extensions explicitly

eglGetProcAddress() returns non-null function pointers
whether or not they're actually supported by the driver,
since it can be used before any driver gets loaded. So
we have to check if the extensions are advertised first,
which requires having an initialized display, so we split
the display creation code into its own function.

The exception to extension-checking is EGL_MESA_drm_display,
since by definition it's needed before any display is even
created.