emscripten: restore key event prevent default behaviour

This restores the behaviour before 92215481146f9225a458e9a09abb85a33b52d9ff where we only preventDefault the event if:
- the key is recognised
- the event is enabled
- the event is not filtered

(ignoring the KEYPRESS special case, which is unchanged)

(cherry picked from commit 6e931bee01b34a9f7a51579bcaf9a95f7f9451ce)

src/video/emscripten/SDL_emscriptenevents.c

@@ -458,7 +458,7 @@ static EM_BOOL Emscripten_HandleKey(int eventType, const EmscriptenKeyboardEvent
     SDL_WindowData *window_data = (SDL_WindowData *)userData;
     SDL_Scancode scancode = Emscripten_MapScanCode(keyEvent->code);
     SDL_Keycode keycode = SDLK_UNKNOWN;
-    SDL_bool prevent_default = SDL_TRUE;
+    SDL_bool prevent_default = SDL_FALSE;
     SDL_bool is_nav_key = SDL_FALSE;
 
     if (scancode == SDL_SCANCODE_UNKNOWN) {
@@ -511,9 +511,9 @@ static EM_BOOL Emscripten_HandleKey(int eventType, const EmscriptenKeyboardEvent
     }
 
     if (keycode != SDLK_UNKNOWN) {
-        SDL_SendKeyboardKeyAndKeycode(0, SDL_DEFAULT_KEYBOARD_ID, 0, scancode, keycode, eventType == EMSCRIPTEN_EVENT_KEYDOWN ? SDL_PRESSED : SDL_RELEASED);
+        prevent_default = SDL_SendKeyboardKeyAndKeycode(0, SDL_DEFAULT_KEYBOARD_ID, 0, scancode, keycode, eventType == EMSCRIPTEN_EVENT_KEYDOWN ? SDL_PRESSED : SDL_RELEASED);
     } else {
-        SDL_SendKeyboardKey(0, SDL_DEFAULT_KEYBOARD_ID, 0, scancode, eventType == EMSCRIPTEN_EVENT_KEYDOWN ? SDL_PRESSED : SDL_RELEASED);
+        prevent_default = SDL_SendKeyboardKey(0, SDL_DEFAULT_KEYBOARD_ID, 0, scancode, eventType == EMSCRIPTEN_EVENT_KEYDOWN ? SDL_PRESSED : SDL_RELEASED);
     }
 
     /* if TEXTINPUT events are enabled we can't prevent keydown or we won't get keypress