ui/sdl2: set swap interval explicitly when OpenGL is enabled

Before 176e378 (ui/sdl2: OpenGL window context)
SDL_CreateRenderer was called unconditionally setting
the swap interval to 0. Since SDL_CreateRenderer is now no
longer called when OpenGL is enabled, the swap interval is
no longer set explicitly and vsync handling depends on
the environment settings which may lead to a performance
regression with virgl as reported in
   https://gitlab.com/qemu-project/qemu/-/issues/2565

Restore the old vsync handling by explicitly calling
SDL_GL_SetSwapInterval if OpenGL is enabled.

Fixes: 176e378 (ui/sdl2: OpenGL window context)
Closes: https://gitlab.com/qemu-project/qemu/-/issues/2565

Signed-off-by: Gert Wollny <[email protected]>
Acked-by: Marc-André Lureau <[email protected]>
Message-ID: <01020191e05ce6df-84da6386-62c2-4ce8-840e-ad216ac253dd-000000@eu-west-1.amazonses.com>
Signed-off-by: Philippe Mathieu-Daudé <[email protected]>
(cherry picked from commit ae23cd0)
Signed-off-by: Michael Tokarev <[email protected]>

ui/sdl2.c

@@ -115,6 +115,7 @@ void sdl2_window_create(struct sdl2_console *scon)
         SDL_SetHint(SDL_HINT_RENDER_BATCHING, "1");
 
         scon->winctx = SDL_GL_CreateContext(scon->real_window);
+        SDL_GL_SetSwapInterval(0);
     } else {
         /* The SDL renderer is only used by sdl2-2D, when OpenGL is disabled */
         scon->real_renderer = SDL_CreateRenderer(scon->real_window, -1, 0);