testmessage: Create a renderer for window display on Wayland

On Wayland -- or at least on some Wayland implementations -- windows
aren't shown until something has been rendered into them. For the
'testmessage' test program, this means that the final messagebox (a
modal one) is blocking an "invisible window", which can then be
difficult to close.

By creating a renderer and presenting once, the window is properly
displayed, and the test behaves as it does under X11 (including
XWayland).

test/testmessage.c

@@ -189,6 +189,12 @@ main(int argc, char *argv[])
         SDL_Event event;
         SDL_Window *window = SDL_CreateWindow("Test", SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED, 640, 480, 0);
 
+        /* On wayland, no window will actually show until something has
+           actually been displayed.
+        */
+        SDL_Renderer *renderer = SDL_CreateRenderer(window, -1, 0);
+        SDL_RenderPresent(renderer);
+
         success = SDL_ShowSimpleMessageBox(SDL_MESSAGEBOX_ERROR,
                     "Simple MessageBox",
                     "This is a simple error MessageBox with a parent window",