Emscripten: Partially fixed not removing key event callbacks if hint was used.

The key event callbacks were always removed from the same target although it is
possible to set them to different targets using the hint. This is only a partial
fix because it assumes that the hint is not changed to a different value later.

src/video/emscripten/SDL_emscriptenevents.c

@@ -621,10 +621,15 @@ Emscripten_UnregisterEventHandlers(SDL_WindowData *data)
     emscripten_set_touchmove_callback("#canvas", NULL, 0, NULL);
     emscripten_set_touchcancel_callback("#canvas", NULL, 0, NULL);
 
-    emscripten_set_keydown_callback("#window", NULL, 0, NULL);
-    emscripten_set_keyup_callback("#window", NULL, 0, NULL);
+    const char *target = SDL_GetHint(SDL_HINT_EMSCRIPTEN_KEYBOARD_ELEMENT);
+    if (!target) {
+        target = "#window";
+    }
+
+    emscripten_set_keydown_callback(target, NULL, 0, NULL);
+    emscripten_set_keyup_callback(target, NULL, 0, NULL);
 
-    emscripten_set_keypress_callback("#window", NULL, 0, NULL);
+    emscripten_set_keypress_callback(target, NULL, 0, NULL);
 
     emscripten_set_fullscreenchange_callback("#document", NULL, 0, NULL);