cogl-winsys-egl: Use the abstraction to update the window size

Previously the EGL backend was directly prodding the width/height
members of the framebuffer structure when a configure notify event is
received. However this doesn't set the dirty flag for the viewport so
Cogl will continue using the wrong viewport y offset. The GLX backend
is already using an abstraction for updating the size which does set
the flag. This patch just makes the EGL backend also use that
abstraction.

cogl/winsys/cogl-winsys-egl.c

@@ -275,9 +275,9 @@ event_filter_cb (XEvent *xevent, void *data)
         {
           CoglFramebuffer *framebuffer = COGL_FRAMEBUFFER (onscreen);
 
-          /* XXX: consider adding an abstraction for this... */
-          framebuffer->width = xevent->xconfigure.width;
-          framebuffer->height = xevent->xconfigure.height;
+          _cogl_framebuffer_winsys_update_size (framebuffer,
+                                                xevent->xconfigure.width,
+                                                xevent->xconfigure.height);
         }
     }