sdl2: restore window dimensions by resize

sdl2: restore window dimensions by resize

instead of destroying and recreating window, fixes segfault caused by
handle_keyup trying to access no more existing window when using
Ctrl-Alt-U to restore window "un-scaled" dimensions

 Thread 1 "qemu-system-x86" received signal SIGSEGV, Segmentation fault.
 [Switching to Thread 0x7ffff7f92b80 (LWP 3711)]
 handle_keyup (ev=0x7fffffffd010) at ui/sdl2.c:416
 416         scon->ignore_hotkeys = false;
 (gdb) bt
 #0  handle_keyup (ev=0x7fffffffd010) at ui/sdl2.c:416
 #1  sdl2_poll_events (scon=0x100fee5a8) at ui/sdl2.c:608
 #2  0x0000000100585bf2 in dpy_refresh (s=0x101ad3e00) at ui/console.c:1658
 #3  gui_update (opaque=0x101ad3e00) at ui/console.c:205
 #4  0x0000000100690f2c in timerlist_run_timers (timer_list=0x100ede130) at util/qemu-timer.c:536
 #5  0x0000000100691177 in qemu_clock_run_timers (type=QEMU_CLOCK_REALTIME) at util/qemu-timer.c:547
 #6  qemu_clock_run_all_timers () at util/qemu-timer.c:674
 #7  0x0000000100691651 in main_loop_wait (nonblocking=<optimized out>) at util/main-loop.c:503
 #8  0x00000001003d650f in main_loop () at vl.c:1848
 #9  0x0000000100289681 in main (argc=<optimized out>, argv=<optimized out>, envp=<optimized out>) at vl.c:4605

Signed-off-by: Amadeusz Sławiński <amade@asmblr.net>
Message-id: 20180613172707.31530-1-amade@asmblr.net
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>