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

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

Before 176e3783f2ab (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: 176e3783f2ab (ui/sdl2: OpenGL window context)
Closes: https://gitlab.com/qemu-project/qemu/-/issues/2565
Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Acked-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-ID: <01020191e05ce6df-84da6386-62c2-4ce8-840e-ad216ac253dd-000000@eu-west-1.amazonses.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>