Capture Alt and Meta keys

Alt and Meta keys should not be forwarded to the device. For now, they
are not used for shortcuts, but they could be.

app/src/input_manager.c

@@ -151,6 +151,14 @@ void input_manager_process_text_input(struct input_manager *input_manager,
 void input_manager_process_key(struct input_manager *input_manager,
                                const SDL_KeyboardEvent *event) {
     SDL_bool ctrl = event->keysym.mod & (KMOD_LCTRL | KMOD_RCTRL);
+    SDL_bool alt = event->keysym.mod & (KMOD_LALT | KMOD_RALT);
+    SDL_bool meta = event->keysym.mod & (KMOD_LGUI | KMOD_RGUI);
+
+    if (alt | meta) {
+        // no shortcut involves Alt or Meta, and they should not be forwarded
+        // to the device
+        return;
+    }
 
     // capture all Ctrl events
     if (ctrl) {