sdl: Do not disable screensaver by default

Unless we are running in full-screen mode, QEMU's SDL window should not
disable the host's screensaver. The user can still change this behaviour
by setting the environment variable SDL_VIDEO_ALLOW_SCREENSAVER as
desired.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

sdl.c

diff --git a/sdl.c b/sdl.c
index 16a48e9..3bdd518 100644
--- a/sdl.c
+++ b/sdl.c
@@ -855,6 +855,10 @@
     if (no_frame)
         gui_noframe = 1;
 
+    if (!full_screen) {
+        setenv("SDL_VIDEO_ALLOW_SCREENSAVER", "1", 0);
+    }
+
     flags = SDL_INIT_VIDEO | SDL_INIT_NOPARACHUTE;
     if (SDL_Init (flags)) {
         fprintf(stderr, "Could not initialize SDL(%s) - exiting\n",