examples: Fix potential tearing

The image needs to be switched during the vertical blanking period so
that no tearing is shown during the switch.

examples/graphics/backbuffer2/source/main.c

@@ -114,11 +114,11 @@ int main(int argc, char **argv)
             }
         }
 
-        // Swap backbuffer and visible buffers
-        NF_Flip16bitsBackBuffer(1);
-
         // Wait for the screen refresh
         swiWaitForVBlank();
+
+        // Swap backbuffer and visible buffers
+        NF_Flip16bitsBackBuffer(1);
     }
 
     return 0;