ui: sdl2: Release grab before opening console window

sdl 2.0.4 currently has a bug which causes our UI shortcuts to fire
rapidly in succession:

  https://bugzilla.libsdl.org/show_bug.cgi?id=3287

It's a toss up whether ctrl+alt+f or ctrl+alt+2 will fire an
odd or even number of times, thus determining whether the action
succeeds or fails.

Opening monitor/serial windows is doubly broken, since it will often
lock the UI trying to grab the pointer:

  0x00007fffef3720a5 in SDL_Delay_REAL () at /lib64/libSDL2-2.0.so.0
  0x00007fffef3688ba in X11_SetWindowGrab () at /lib64/libSDL2-2.0.so.0
  0x00007fffef2f2da7 in SDL_SendWindowEvent () at /lib64/libSDL2-2.0.so.0
  0x00007fffef2f080b in SDL_SetKeyboardFocus () at /lib64/libSDL2-2.0.so.0
  0x00007fffef35d784 in X11_DispatchFocusIn.isra.8 () at /lib64/libSDL2-2.0.so.0
  0x00007fffef35dbce in X11_DispatchEvent () at /lib64/libSDL2-2.0.so.0
  0x00007fffef35ee4a in X11_PumpEvents () at /lib64/libSDL2-2.0.so.0
  0x00007fffef2eea6a in SDL_PumpEvents_REAL () at /lib64/libSDL2-2.0.so.0
  0x00007fffef2eeab5 in SDL_WaitEventTimeout_REAL () at /lib64/libSDL2-2.0.so.0
  0x000055555597eed0 in sdl2_poll_events (scon=0x55555876f928) at ui/sdl2.c:593

We can work around that hang by ungrabbing the pointer before launching
a new window. This roughly matches what our sdl1 code does

Signed-off-by: Cole Robinson <crobinso@redhat.com>
Message-id: 31c9ab6540b031f7a614c59edcecea9877685612.1462557436.git.crobinso@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
