add RGBA8888 pixel format check in various SDL APIs

Updated SDL_GetRGB/RGBA and SDL_MapRGB/RGBA to check that the pixel format passed as an argument is RGBA8888, the only format supported by the SDL emscripten port. A warning will be shown to the user if that format does not match - in this scenario, the current code still does the mapping anyway (this could be changed to return right away if you prefer). I also fixed an issue with the access to SDL_PIXELFORMAT_RGBA8888 from library_sdl.js as struct_info.json did not have this definition specified.
Please put all the body of this function in `#if ASSERTIONS ... #endif`