lavd/sdl2: add sdl alias

This commit also adds an sdl alias for the configure script.

configure

@@ -1547,6 +1547,7 @@ EXTERNAL_LIBRARY_LIST="
     opengl
     openssl
     schannel
+    sdl
     sdl2
     securetransport
     videotoolbox
@@ -5846,6 +5847,7 @@ if enabled gcrypt; then
     fi
 fi
 
+disabled sdl && disable sdl2
 if ! disabled sdl2; then
     SDL2_CONFIG="${cross_prefix}sdl2-config"
     if check_pkg_config sdl2 SDL_events.h SDL_PollEvent; then

libavdevice/sdl2.c

@@ -352,7 +352,7 @@ static const AVClass sdl2_class = {
 };
 
 AVOutputFormat ff_sdl2_muxer = {
-    .name           = "sdl2",
+    .name           = "sdl,sdl2",
     .long_name      = NULL_IF_CONFIG_SMALL("SDL2 output device"),
     .priv_data_size = sizeof(SDLContext),
     .audio_codec    = AV_CODEC_ID_NONE,