video/x11: improve keyboard mapping

Handle mapping for keycodes not associated with a Unicode character.
---
 src/video/x11/SDL_x11keyboard.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/src/video/x11/SDL_x11keyboard.c b/src/video/x11/SDL_x11keyboard.c
index 53866ab..cdf6fb9 100644
--- a/src/video/x11/SDL_x11keyboard.c
+++ b/src/video/x11/SDL_x11keyboard.c
@@ -317,6 +317,9 @@ X11_UpdateKeymap(_THIS)
         if (key) {
             keymap[scancode] = key;
         }
+        else {
+            keymap[scancode] = SDL_SCANCODE_TO_KEYCODE(X11_KeyCodeToSDLScancode(data->display, (KeyCode)i));
+        }
     }
     SDL_SetKeymap(0, keymap, SDL_NUM_SCANCODES);
 }
-- 
2.1.0