Fix wxGLCanvasEGL::IsShownOnScreen() under Wayland

Don't use m_readyToDraw in it, as the canvas is still shown even when
it's not ready to draw yet, and use m_wlSubsurface instead as it is
destroyed when the window is unmapped.

See wxWidgets#23899.

(cherry picked from commit cf44a87)

Author: vadz

src/unix/glegl.cpp

@@ -851,7 +851,7 @@ bool wxGLCanvasEGL::IsShownOnScreen() const
         case wxDisplayX11:
             return GetXWindow() && wxGLCanvasBase::IsShownOnScreen();
         case wxDisplayWayland:
-            return m_readyToDraw && wxGLCanvasBase::IsShownOnScreen();
+            return m_wlSubsurface && wxGLCanvasBase::IsShownOnScreen();
         case wxDisplayNone:
             break;
     }