video: Disable text input on startup.

The most recent versions of SDL2 seem to start up the game with text
input enabled. We don't want this; text input should only be enabled
if we're typing something in (= server address for a network game).
Having it enabled like this breaks the key rebindings screen.

Fixes bug reported on the Sopwith Discord by ranguli; thanks also to
NY00123 for investigations.

src/sdl/video.c

@@ -380,6 +380,8 @@ static void Vid_SetMode(void)
 		error_exit("Failed to open SDL window: %s", SDL_GetError());
 	}
 
+        SDL_StopTextInput();
+
         pixel_format = SDL_GetWindowPixelFormat(window);
 
 	for (n = 0; n < NUM_KEYS; ++n) {