Property SDL_PROP_WINDOW_EMSCRIPTEN_CANVAS_ID missing from SDL_GetWindowProperties (Emscripten)

[vittorioromeo]

One more Emscripten inconsistency that might become more relevant when #12575 is finalized. It is currently possible to set SDL_PROP_WINDOW_CREATE_EMSCRIPTEN_CANVAS_ID when creating a window via SDL_CreateWindowWithProperties:

SDL_SetNumberProperty(props, SDL_PROP_WINDOW_CREATE_EMSCRIPTEN_CANVAS_ID, canvasId);

But it is not possible to retrieve that canvas id anymore via SDL_GetWindowProperties + SDL_GetNumberProperty, as the SDL_PROP_WINDOW_EMSCRIPTEN_CANVAS_ID does not exist.