sdl/consumer_sdl.c: use mutex when changing sdl_screen

sdl/consumer_sdl.c: use mutex when changing sdl_screen

Fixes Coverity CID 709357 and 709358: Data race condition (MISSING_LOCK)

Locking "mlt_sdl_mutex".
235                        pthread_mutex_lock( &mlt_sdl_mutex );
consumer_sdl_s.sdl_screen is being accessed with lock "mlt_sdl_mutex" held.
236                        this->sdl_screen = SDL_GetVideoSurface( );
237                        pthread_mutex_unlock( &mlt_sdl_mutex );
...
Locking "mlt_sdl_mutex".
269                        pthread_mutex_lock( &mlt_sdl_mutex );
consumer_sdl_s.sdl_screen is being accessed with lock "mlt_sdl_mutex" held.
270                        this->sdl_screen = SDL_SetVideoMode( this->window_width, this->window_height, 0, this->sdl_flags );
271                        pthread_mutex_unlock( &mlt_sdl_mutex );
...
Accessing variable "this->sdl_screen" (consumer_sdl_s.sdl_screen) requires the mlt_sdl_mutex lock.
315                this->sdl_screen = NULL;
...
Locking "mlt_sdl_mutex".
573                        pthread_mutex_lock( &mlt_sdl_mutex );
consumer_sdl_s.sdl_screen is being accessed with lock "mlt_sdl_mutex" held.
574                        this->sdl_screen = SDL_SetVideoMode( this->window_width, this->window_height, this->bpp, this->sdl_flags );
575                        if ( consumer_get_dimensions( &this->window_width, &this->window_height ) )
consumer_sdl_s.sdl_screen is being accessed with lock "mlt_sdl_mutex" held.
576                                this->sdl_screen = SDL_SetVideoMode( this->window_width, this->window_height, this->bpp, this->sdl_flags );
577                        pthread_mutex_unlock( &mlt_sdl_mutex );