vgl: use initial size

This change causes a duplicated invocation of the resize at start-up.
That should be a no-op, but in any case, it will be deduplicated with
the following two changes. Together, they make no functional changes.

modules/video_output/vgl.c

@@ -137,7 +137,7 @@ static int Open(vlc_gl_t *gl, unsigned width, unsigned height)
             return VLC_EGENERIC;
         }
 
-    (void) width; (void) height;
+    Resize(gl, width, height);
     return VLC_SUCCESS;
 }