sdl2: Fix broken display updating after the window is hidden

With SDL 2.0.6, calling SDL_ShowWindow during SDL_WINDOWEVENT_HIDDEN
blocks all subsequent display updates.

Instead of trying to override the change, just update the scon->hidden
flag.

Signed-off-by: Jindrich Makovicka <makovick@gmail.com>
Message-Id: <20171112193032.9724-2-makovick@gmail.com>

This is a partial revert of d3f3a0f453ea590be529079ae214c200bb5ecc1a,
which in turn is a workaround for a SDL bug.  The bug is fixed in 2.0.6,
see https://bugzilla.libsdl.org/show_bug.cgi?id=3410

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
