egl: Enable EGL_EXT_client_extensions

Insert two fields into _egl_global to hold the client extensions and
statically initialize them:
    ClientExtensions // a struct of bools
    ClientExtensionString

Post-patch, Mesa supports exactly one client extension,
EGL_EXT_client_extensions.

Signed-off-by: Chad Versace <chad.versace@linux.intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/egl/main/eglglobals.c

@@ -47,6 +47,14 @@ struct _egl_global _eglGlobal =
       _eglUnloadDrivers, /* always called last */
       _eglFiniDisplay
    },
+
+   /* ClientExtensions */
+   {
+      true /* EGL_EXT_client_extensions */
+   },
+
+   /* ClientExtensionsString */
+   "EGL_EXT_client_extensions"
 };